What is netcode?

There is your client machine, and there are other client machines, and there is the server.

Coordinating all of what is going on among all the client machines and the server is with netcode

Basically, there is an instantaneous "state" of the game on your machine, and on the server, and on all client machines.

Ideally this state would be the same on all involved machines.  But it's not.  Because there is network lag, and there is inferior netcode that can't keep up synchronizing the state with how fast things are happening.

So you see crazy looking stuff.