An honest node is one that conforms to our expected node behavior. This means that nodes do not attempt to modify history, serve blocks and transactions correctly, transmit messages correctly, transmit formatted messages and data correctly, etc. These all have the right way to operate. Honest nodes are the ones who do these operations correctly.
This is in contrast to malicious nodes, whose behavior does not conform to our expected node behavior. Such nodes may send malformed messages, send incorrect data, send too much or too little data, and so on. These are wrong versions of what the nodes are supposed to do, so the behavior of these nodes is not the behavior we expect from nodes.