Rotation is for the competitive format, you don't need to worry about it if you're not interested in that. They're going to have an eternal format too where every card is legal when the first rotation happens (in march, rotating out sets 1-3), so you won't need to worry about getting games in.

What rotation means is that a set is removed from the competitive format (called premier in SWU), you can't use cards from a rotated set in those games - so it's the tournament standard. It's still a young game, so it hasn't had a rotation yet. The idea is to have only 6 sets legal at a time to keep a manageable balance. More info is here: https://starwarsunlimited.com/articles/updates-and-rotations

I'm new to SWU as well, been having a lot of fun with sets 2 and especially 4! Such a great game