In layman terms, the netcode is just the code the game use to run while connected with others via the internet. It doesn't suck by itself, because afterall the netcode is what connects between players after all. What sucks is the way the developers use it.

One of the ways to do it is the "input delay" (what this image is calling delay based netcode), which means that any action that you do when pressing buttons will happen a little bit later than what normally could happen offline, because the game checks you and the other player are synchronized first.

So, one of the most popular alternatives is using Rollback netcode, but I guess that's more than what you are asking, so I'll look to some videos that can explain better what I said and a little bit more if you are interested.

Here's a small video comparing between input delay vs rollback: