Its pretty easy. My daughter has been playing netball for a while and we attended a couple of Celtic Dragons games last season. I only know the basic rules from watching my daughter play but i appreciated the skill on show and the event. Obviously the more you see the more you enjoy.
I have played and followed Cricket all my life and the T20 games are practically nights out and we always take down a good group some are not interested in the game at all.
There needs to a base interest there, otherwise you will go and just be looking at your phone all day, but there are no rules that people need to have a base knowledge to enjoy something, even in football.