In Ethereum, the chain ID is used to determine which chain a transaction should be processed on. For example, if a transaction is meant to be processed on the Ethereum mainnet, the chain ID would be 1.
If the transaction is meant to be processed on a testnet, the chain ID would be 3.
The chain ID is also used when signing transactions. When a transaction is signed, the chain ID is included in the signature.
This allows for replay protection, so that if a transaction is meant for one chain but ends up being broadcast on another chain, it will not be valid.
The chain ID is an important part of Ethereum and helps keep the network secure.