Nodeos generally runs in two modes:
Producing Nodes are configured for block production. They connect to the peer-to-peer network and actively produce new blocks. Loose transactions are also validated and relayed. On mainnet, Producing Nodes only produce blocks if their assigned block producer is part of an active schedule.
Non-Producing Nodes connect to the peer-to-peer network but do not actively produce new blocks; they are useful for acting as proxy nodes, relaying API calls, validating transactions, broadcasting information to other nodes, etc. Non-Producing Nodes are also useful for monitoring the blockchain state.