A Partially Signed Bitcoin Transaction (PSBT) is a bitcoin standard that facilitates portability of unsigned transactions, which allows multiple parties to easily sign the same transaction. This is most useful when multiple parties wish to add inputs to the same transaction.

Imagine several board members of a corporation are all required to contribute funds to a transaction, and would like to do so in one transaction, to ensure that no member bails out (and to save on fees). PSBT allows all members to easily sign their portion of the transaction without worrying that the transaction could be changed in transit or that the funds could be stolen.