This is about how we trust the "book-keepers" and admins of transactions, how you have trust in a decentralised system. PoW forces the book-keepers to expend energy in a digital lottery where they try to guess the right number, competing against all the other book-keepers who are guessing as well; if a book-keeper tries to censor transactions, but fails to guess the right number for the block, their energy is wasted, and the transaction can get through any of the other book-keepers who aren't censoring anyone and happen to guess the right number.
That random nature of the contest, and being forced to expend actual "work" that costs something to do, is how you get trust decentralised.