A bridge is a protocol for transferring assets between different blockchains. Blockchains are isolated. Bitcoin can't directly transfer to Ethereum. A bridge creates a way to move assets between them. The basic mechanism is locking and minting. You lock assets on Chain A. The bridge mints equivalent assets on Chain B. Later you can burn the assets on Chain B and unlock the originals on Chain A.
For example, bridge bitcoin to Ethereum as WBTC. You lock BTC with a custodian. They mint WBTC on Ethereum. You trade WBTC on Ethereum, use it in DeFi protocols. When you want real BTC back, you burn WBTC and the custodian releases BTC. Bridges are essential for cross-chain DeFi. You can access Ethereum's biggest lending protocols, trading protocols, and yield opportunities from any blockchain.
The downside is that bridges are frequent attack targets. Custody is the weakest link. If the custodian is compromised or malicious, the bridge loses funds. Most major bridge hacks have been custody failures. Bridges are improving. Multi-signature custody spreads risk. Zk-proofs reduce reliance on trusted validators. But bridge risk remains.
Using bridges means trusting the bridge's security model.