In hindsight it's about a high school glee club. The glee club used to be the "top dogs" of this school but now they've sunk into relative obscurity, and a teacher who used to be part of this glee club is trying to restore it to its former glory.
But in reality, it's much more than that. The show itself is drama mixed with comedy mixed with musical. The main story revolves around the glee club, but it's more than that. Teen pregnancy, fake pregnancy, drugs, peer pressure, sports, deception and lies, love and lust, discrimination...there's so much going on, and because of this anyone can get some entertainment out of it. It also sends very inspirational messages, and that's why the critics love it.
There's a lot of things good about Glee, but from the inside looking out, it's hard to explain. As Matthew Morrison, one of the actors from the show said, you can't compare it to anything else. It's fresh, it's original, and it throws things at you that you have never or rarely seen in TV.
I recommend you watch it. I guarantee you won't be disappointed. It's a lot of fun.