V sadě lepení jsou dvě záplaty.

Because the preposition v meaning in normally takes the locative case.

The noun sada changes like this:

• sada = nominative singular
• v sadě = locative singular

So v sadě means in the kit / in the set.

Lepení is a verbal noun from lepit, meaning to glue, to stick, or in this kind of context to patch / repair with patches.

So sada lepení is trying to mean something like a patching kit or a repair-gluing kit.

One important note: this phrasing sounds a bit compressed. In more natural everyday Czech, you would very often hear:

• sada na lepení
• opravná sada

So V sadě na lepení jsou dvě záplaty would sound more standard.

After sada, Czech often uses the genitive, so lepení is functioning as genitive singular here: literally something like a set of patching/gluing.

This can be hard to notice because nouns like lepení have the same form in several singular cases. So even though the form does not visibly change, its grammatical role still does.

That said, as mentioned above, sada na lepení is usually more idiomatic than sada lepení.

Because Czech does not use a dummy subject like English there.

English says:

• There are two patches in the kit.

Czech simply uses the verb to be plus normal word order:

• V sadě lepení jsou dvě záplaty.

A more literal English structure would be:

• In the kit are two patches.

So the idea of there are is expressed by the sentence structure itself, not by a separate word.

Because jsou is the 3rd person plural form of být (to be), and the subject is plural:

• dvě záplaty = two patches

Since there are two patches, Czech uses the plural verb:

• jsou = are

Using je here would be wrong for this sentence.

Because the numeral two changes according to grammatical gender.

• dva is used with masculine nouns
• dvě is used with feminine and neuter nouns

The noun záplata is feminine, so you must say:

• dvě záplaty

Compare:

• dva stoly = two tables
• dvě auta = two cars
• dvě záplaty = two patches

Here záplaty is in the nominative plural, because it is the subject of the sentence.

• záplata = singular
• záplaty = plural

In this sentence:

• jsou dvě záplaty = there are two patches

So záplaty is the thing that are in the kit, which makes it the subject.

Yes, Czech word order is quite flexible.

Starting with V sadě lepení puts the location/context first. That is very natural when you are introducing what is inside something.

So this sentence feels like:

• As for the kit, there are two patches in it.

You could rearrange it, but the emphasis would change. For example:

• Dvě záplaty jsou v sadě na lepení.
  This puts more emphasis on two patches.

The most natural fuller version would probably be:

• V sadě na lepení jsou dvě záplaty.

Not in every possible English sense of patch, but it is the right word for a repair patch.

For example, záplata can mean:

• a patch for a hole
• a tyre patch
• a sewn or repair patch

But it is not the usual word for a medical plaster/bandage. That is:

• náplast

So in this sentence, záplaty clearly means repair patches, not medical plasters.