A bridge is a tool that lets assets or information move between separate blockchains, which otherwise cannot communicate directly. For example, a bridge might let you move a token from Ethereum to another network so you can use it in that ecosystem.
Bridges usually work by locking an asset on the original chain and issuing a matching representation on the destination chain. When you want to move back, the process reverses. This allows value to flow between ecosystems that were not designed to connect.
Bridges have been one of the more security-sensitive parts of crypto. Because they hold large amounts of locked assets and rely on complex code, they have been targeted in some of the largest exploits in the space. Understanding this is important context whenever bridging is discussed.