What does node mean?
A computer that possesses a copy of the blockchain and is working to maintain it.
Full nodes are nodes that maintain a full blockchain with all transactions. In the early years of bitcoin, all nodes were full nodes and currently, the Bitcoin Core client is a full blockchain node.
Full nodes are necessary to maintain a secure network, the more working nodes, the more secure Bitcoin becomes. Recent years have seen the number of full nodes decline, as the increased number of cryptocurrency projects has sent people from Bitcoin to other projects.