The first act break is a commitment: the protagonist chooses to engage, and the choice has a cost that makes reversal expensive. The second is usually a collapse: the approach that has driven the middle stops working, and the protagonist has to become someone else or fail. Everything between them is act two, which is why a story with a weak second break has a middle that will not end.
In episodic and broadcast work the break is also literally where the ads go, which is why the beat immediately before it is written to be unresolved. That commercial pressure produced a genuinely useful craft rule: end the unit on a question, not a period.
The test for whether you have a real act break is whether the protagonist could go back. If they could, you have a turning point. If going back would cost them more than going forward, you have an act break.
What is the difference between an act break and a turning point?
Every act break is a turning point; most turning points are not act breaks. An act break changes the rules the story operates under and makes retreat costly. A turning point only redirects the current pursuit.
How many act breaks does a story have?
Two, in three-act structure: the commitment at the end of act one and the collapse at the end of act two. More than two usually means the middle has been divided rather than structured.