Ich lasse das Brot auf dem Tisch auftauen.

Because lassen + infinitive means to let something happen or to leave something to happen.

So:

• Ich taue das Brot auf. = I am defrosting/thawing the bread.
• Ich lasse das Brot auftauen. = I let the bread thaw / I leave the bread to thaw.

The second version suggests less direct action: you are not actively doing much, just allowing the bread to thaw.

Here, lassen means something like:

• let
• allow
• leave

In this sentence, it is very close to leave:

• Ich lasse das Brot auf dem Tisch auftauen. = I leave the bread on the table to thaw.

So lassen does not always mean the same thing in every sentence. It is a very common German verb with several uses, and this is one of the most important ones.

Because auftauen is an infinitive depending on the conjugated verb lasse.

In a main clause, the conjugated verb usually comes in second position, and any dependent infinitive goes to the end:

• Ich lasse das Brot auf dem Tisch auftauen.

This is a very common pattern in German with verbs like:

• wollen
• können
• müssen
• lassen

Auftauen is a separable verb.

When a separable verb is used as a conjugated main verb, the prefix separates:

• Ich taue das Brot auf.

But when it appears as an infinitive, the prefix stays attached:

• Ich lasse das Brot auftauen.

So both forms belong to the same verb:

• auftauen = infinitive
• taue ... auf = finite/conjugated form in a main clause

Because this phrase describes a location, not a direction.

German often uses:

• dative for where?
• accusative for where to?

Here the bread is already on the table, so it is a location:

• auf dem Tisch = on the table

If you were moving it onto the table, you would use accusative:

• Ich lege das Brot auf den Tisch. = I put the bread onto the table.

So:

• auf dem Tisch = where it is
• auf den Tisch = where it goes

Because it is the direct object of the action.

What am I letting thaw?

• das Brot

That makes it the accusative object. In this sentence, lassen takes an object plus an infinitive:

• Ich lasse das Brot auftauen.

This pattern is very common:

• Ich lasse das Fenster offen.
• Ich lasse das Auto reparieren.
• Ich lasse die Suppe kochen.

In each case, the thing affected is in the accusative.

Yes, in many contexts you can, but auftauen is usually the more natural verb when something frozen becomes unfrozen.

Very roughly:

• tauen can mean to thaw
• auftauen often specifically means to defrost / thaw out

For frozen food, auftauen is usually the clearer and more idiomatic choice.

The meaning is similar, but the perspective is different.

• Ich lasse das Brot auf dem Tisch auftauen.
  • focuses on what I am doing: I am leaving it there to thaw.
• Das Brot taut auf dem Tisch auf.
  • focuses on what the bread is doing: the bread is thawing on the table.

So the first sentence includes the person's choice or intention; the second just states the process.

Not really in the everyday sense of permission.

With people, lassen can mean let/allow:

• Ich lasse ihn gehen. = I let him go.

But with things, it often means leave or have something happen:

• Ich lasse das Brot auftauen.
• Ich lasse die Tür offen.

So here it is not about giving permission to the bread. It is about leaving it in a state or situation.

You probably could in some contexts, but das Brot is more natural if you mean a specific piece/loaf of bread.

• das Brot = the bread / this bread
• Brot without an article can sound more general, like bread as a substance

In everyday German, if you are talking about a particular item taken out of the freezer, das Brot fits better.

Yes, but the conjugated verb still has to stay in second position in a normal main clause, and auftauen still stays at the end.

For example:

• Ich lasse das Brot auf dem Tisch auftauen.
• Auf dem Tisch lasse ich das Brot auftauen.

Both are possible. The second version emphasizes on the table.

What does not change in a normal statement is:

• lasse in second position
• auftauen at the end

Yes, it is natural.

It sounds like someone is saying they are leaving the bread on the table so it can thaw. It is an ordinary, idiomatic use of lassen + infinitive and a very normal use of auf dem Tisch for location.