What does a console to pc port actually involve work wise?

Idk how to explain further, I know virtually zero about coding and game design. But I'm just wondering what the actual process involves for a console game being ported to PC or vice versa. It seems like it's complicated, as many developers take a long time to get a pc version out after a console release, or vice versa, and it's hard for me to figure out what exactly is involved.