Blockchain bridges allows you to port assets from one chain onto another, allowing those assets to represent their value and be used on a different chain. Most blockchains are not compatible with each and blockchain bridges bring much needed interoperability between chains by allowing for the psuedo-transfer of assets from one to the other.
Blockchain bridges work by first locking up the asset on its home chain, done by sending the asset to a designated address or smart contract. Once this lockup has been confirmed the protocol or company then mints a synthetic asset, representing the locked asset, on the target chain. The locking up process provides assurance that the asset on the target chain is always backed by the asset on the original chain.
These bridges can be both one-way and bidirectional. One-way bridges only allow you can to send assets in one direction e.g. from Ethereum to Cardano, and not from Cardano to Ethereum. Note that you can always unlock the locked up asset by returning synthetic asset to the bridge. One-way, in this sense means that you cannot lock up assets on the other chain, Cardano in our example, to be used as synthetic assets at the other end of the bridge. Bidirectional blockchain bridges allow you mint synthetic assets on both chains.
Blockchain bridges can be custodial, where the assets are stored by a trusted 3rd party, or non-custodial, where the assets are locked up in a smart contract. Non-custodial blockchain bridges, utilizing code and smart contracts to secure assets instead of a middleman, are championed as being more aligned with the ethos of decentralization.