Learn Dutch: Ik zet het montuur af omdat het een stuk zwaarder is. - I take off the frame because it is a lot heavier.

The phrase een stuk literally means a piece, but before a comparative adjective like zwaarder (heavier), it acts as an intensifier meaning a lot or quite a bit. You could also say veel zwaarder, which means exactly the same thing. Both are very common in everyday Dutch.

Why is the verb is at the very end of the sentence?

The conjunction omdat (because) always introduces a subordinate clause. In Dutch, this means the conjugated verb, which is is in this case, gets kicked to the very end of that specific clause.

Why are zet and af separated in the first half of the sentence?

The infinitive verb is afzetten, which means to take off. It is a separable verb. In a normal main clause, the core verb zet stays in the second position, while the prefix af moves to the end of that clause, just before the conjunction omdat starts the next part.

What kind of frame does het montuur usually refer to?

While it can mean a general framework, het montuur almost always refers to the frame of a pair of glasses. In this context, the speaker is taking off their glasses because the frame feels too heavy.