A Bitcoin full node is a program that fully validates transactions and blocks in the Bitcoin blockchain. It does this by downloading every block and transaction, and then validating them against Bitcoin’s consensus rules. This ensures that all users of the Bitcoin network are following the same rules and that no one is trying to cheat the system.
Full nodes are an important part of the Bitcoin network as they provide a way to ensure that transactions are valid and that blocks are not being manipulated. Full nodes also provide a way for miners to connect to the Bitcoin network and begin mining.