ELI5

A PSBT, or Partially Signed Bitcoin Transaction, is like a "to-do list" for a group of people who are sending some Bitcoin together.

Imagine you're with a few friends, and you all want to chip in to buy a gift. But before paying everyone has to agree and sign their name on the bill. You create a list of who owes what and where the money is going (this is the PSBT).

Now, each friend can look at the list, say, "yep, this looks right", and sign it. But until everyone signs it, the money won't actually go to buy the gift. So the PSBT is kind of like an unfinished agreement that needs everyone's signature before it becomes a real transaction.