Mein Fahrradschloss ist schwer, aber ohne Fahrradschloss lasse ich mein Rad nicht am Bahnhof stehen.

Because mein here is a possessive determiner, like my in English, and it comes directly before the noun:

• mein Fahrradschloss = my bike lock

In the nominative and accusative neuter singular, the form is mein.

Fahrradschloss is a neuter noun: das Fahrradschloss.
So:

• mein Fahrradschloss ist schwer
• ohne mein Fahrradschloss ...

You would only use something like meines in different grammatical situations, for example in the genitive:

• die Farbe meines Fahrradschlosses = the color of my bike lock

German very commonly builds compound nouns by joining words together.

Here:

• Fahrrad = bicycle
• Schloss = lock

So:

• Fahrradschloss = bicycle lock / bike lock

This is extremely normal in German. The last part of the compound determines the grammatical gender, plural, and basic noun type. Since das Schloss is neuter, das Fahrradschloss is also neuter.

Yes, in this sentence schwer most naturally means heavy.

So:

• Mein Fahrradschloss ist schwer = My bike lock is heavy

German schwer can also mean difficult in other contexts, just like English hard/heavy can shift meaning depending on context. But here, because we are talking about a lock, the physical meaning heavy is the intended one.

German word order is flexible, but in a main clause the finite verb must be in second position.

The second clause is:

• aber ohne Fahrradschloss lasse ich mein Rad nicht am Bahnhof stehen

The first element placed before the verb is:

• ohne Fahrradschloss

Then the verb comes second:

• lasse

Then the subject comes after the verb:

• ich

So the structure is:

• [aber] [ohne Fahrradschloss] [lasse] [ich] ...

This is normal German word order. If you started with the subject instead, you could also say:

• aber ich lasse mein Rad ohne Fahrradschloss nicht am Bahnhof stehen

That is also grammatical, though the emphasis is slightly different.

All three are possible, but they do not mean exactly the same thing.

In the sentence:

• ohne Fahrradschloss

the noun is used in a general sense:

• without a bike lock

This sounds natural when talking about the idea of not having any lock with you.

Compare:

• ohne Fahrradschloss = without a bike lock / without any bike lock
• ohne ein Fahrradschloss = without a bike lock (possible, but less idiomatic here)
• ohne mein Fahrradschloss = without my bike lock specifically

Also, ohne takes the accusative case. So if you did use a determiner, you would get:

• ohne mein Fahrradschloss
• ohne das Fahrradschloss
• ohne ein Fahrradschloss

Ohne always takes the accusative.

So:

• ohne Fahrradschloss
• ohne mein Fahrradschloss
• ohne das Fahrradschloss

In this sentence, you do not see a visible case ending on the bare noun Fahrradschloss, because neuter singular nouns often look the same in nominative and accusative. But grammatically, it is still accusative after ohne.

Rad is a very common shorter word for bicycle in German.

So:

• das Fahrrad = the bicycle
• das Rad = the bike / bicycle

In everyday language, people often alternate between the two to avoid repetition. That is exactly what is happening here:

• Mein Fahrradschloss ...
• ... lasse ich mein Rad nicht ...

This is very natural German style.

Here lassen means something like to leave or to let remain, and stehen means stand.

Together:

• etwas irgendwo stehen lassen = to leave something standing somewhere

With vehicles like bikes, this often means to leave/park it there.

So:

• ich lasse mein Rad nicht am Bahnhof stehen means:
• I don’t leave my bike at the station or more naturally in context:
• I won’t leave my bike at the station

The two-part structure is important:

• lasse = conjugated verb in second position
• stehen = infinitive at the end

In this sentence, nicht negates the action of leaving the bike there.

• ... lasse ich mein Rad nicht am Bahnhof stehen

A helpful way to hear it is:

• I do not leave my bike standing at the station

German nicht is often placed before the part being negated or near the end of the clause if it negates the whole verbal idea.

Here it comes before the location phrase and infinitive group:

• nicht am Bahnhof stehen

This is very natural. Another word order is also possible in some contexts, but the given version is standard and idiomatic.

Am Bahnhof is a contraction of:

• an dem Bahnhof

So:

• an + dem = am

Literally, an can mean at, by, or on, depending on context.
Here:

• am Bahnhof = at the train station / at the station

This is a fixed, very common expression.

Because German often splits its verb structure in main clauses.

Here the full verbal idea is:

• stehen lassen = to leave standing

But in a main clause, the finite verb goes in second position, while the other verb goes to the end:

• ... lasse ich mein Rad nicht am Bahnhof stehen

This kind of split is very common in German:

• Ich will nach Hause gehen.
• Sie kann gut schwimmen.
• Wir lassen das Auto dort stehen.

So lasse appears early, and stehen appears at the end.

Yes. Aber means but, and it connects the two contrasting ideas:

• My bike lock is heavy, but without a bike lock I don’t leave my bike at the station.

If you remove aber, the sentence still works grammatically, but the contrast becomes less explicit:

• Mein Fahrradschloss ist schwer. Ohne Fahrradschloss lasse ich mein Rad nicht am Bahnhof stehen.

That sounds like two separate statements rather than one clearly contrasted sentence.

Yes, it is in the present tense:

• ist
• lasse

But, as often in both German and English, the present tense can express a general habit, policy, or usual behavior, not just something happening right now.

So the second clause can be understood as:

• I don’t leave my bike at the station without a lock
• I won’t leave my bike at the station without a lock

Both are reasonable English interpretations depending on context.