You could probably just read up on the basic premise to get the gist of it, and then jump in.
It's post apocalyptic sci-fi. Aliens have invaded and wiped out a big chunk of humanity. The show follows a small group of survivors/freedom fighters as they run from and fight against the aliens. The aliens have a way to enslave children by putting a bug thingy on their back that connects to their spines. The group makes it to Charleston where an even bigger group of survivors have managed to create a semi-safe home for themselves, although they are under constant threat of attack, and there are political shenanigans. There are some rebel aliens who have joined the humans, and there are another group of aliens who just showed up at the end of last season who appear to be on the humans side, but that is still a bit of a mystery.
The premise of the show is fantastic, but the writing is terrible at times. Some of the acting is awful, too. The special effects are pretty good, but obviously not big budget.