Bridges are points between two different blockchains, allowing users to move between the two. In reality, like currency in traditional finance (TradFi), tokens typically do not move across blockchains. Rather, the amount changes between blockchains.
Bridges can be either centralised or decentralised.
A centralised bridge is an exchange or platform that supports a digital asset on more than one network. Simply by depositing via one chain and withdrawing via another turns the platform into a centralised bridge. A decentralised bridge operates for this specific purpose — having assets on both networks and taking a commission from users to send from one network in order to receive it at the other.