Bridge
A bridge between two blockchains that moves tokens from A to B.
Each blockchain lives in isolation, so your ETH on Ethereum doesn't exist on Solana. A bridge solves it: it locks your token on the source chain and mints a 'copy' (wrapped) on the destination chain. Most popular are: Wormhole, LayerZero (Stargate), Hop, Across, Synapse. Risk: bridges are the most-hacked spot in crypto, such as the Ronin Bridge (2022, -$625M), Wormhole (-$326M), and Nomad (-$190M).
In plain words
Your ETH on Ethereum doesn't exist on Solana or BSC, as these are separate networks, each with its own state. A bridge locks your token on one side and mints a 'copy' on the other. Use only proven, large bridges, as this is historically the most-hacked spot in crypto, with $2B+ in total losses.
Example
Ronin Bridge (Axie Infinity), Mar 2022: compromise of 5 of 9 validators = $625M loss.