A Bitcoin Core node is a software program that runs the original Bitcoin protocol, which is the backbone of the Bitcoin network. It is responsible for validating and relaying transactions on the network, as well as propagating new blocks to the rest of the network.
The node stores the entire blockchain, which is the ledger of all Bitcoin transactions since its inception. Bitcoin Core nodes have a full copy of the blockchain, which is constantly being updated as new blocks are mined.
The node also helps to secure the network by monitoring the activity of miners and other nodes in the network. The Bitcoin Core node is what allows users to send and receive Bitcoin transactions and provides them with the most secure and reliable way of using the cryptocurrency.