Bridge

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 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 access 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 required for cross-chain DeFi. You can access Ethereum's biggest lending protocols, trading protocols, and yield opportunities from any blockchain.

A downside is 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.