From Transit.Wiki

FasTrak is an electronic highway toll payment system for the State of California. Drivers attach a transponder to the windsheid of the vehicle and toll is collected electronically at toll booths or express lane entrances. It is a debit system so users need to put money into the FasTrak account with cash (or link the account to a debit or credit card) in order to use it.

FasTrak users save time from having to wait and pay at tollbooths. It is also the preferred method to pay tolls on the Golden Gate Bridge, carpool lanes for other Bay Area toll bridges, toll highways in Orange County, and express lanes throughout California.