it's a temporary payment that gets returned when certain obligations are met (often at the end of trial when you're either free to go or going to prison). Bail also often comes with conditions like an ankle monitor, drug/alcohol testing, staying within a certain state... etc. Part of the idea is that you want that money back so you'll be on your best behavior and comply.

Tangentially, it's really important to be out on bail while your court case progresses. Access to lawyers, income, friends, support, research, etc is a critical factor in preparing your defense.

Here's a fun paper https://www.journals.uchicago.edu/doi/abs/10.1086/695285