I hear it all the time at the end of a sentence in anime but what is the meaning of it?
Thanks
ではない - is not
じゃない - casual, abbreviated version of ではない
じゃねえ - even more casual, kind of slurred version of じゃない
が - conjunction meaning “but”
Are you sure it isn’t じゃねえか？ 
You’re probably right. Sorry, I keep hearing が but it’s probably か
じゃない it’s not
じゃないか is it not?
いいんじゃねえか？ It’s fine, isn’t it?
Careful with じゃない, because while it literally means “not,” it’s used all the time as an affirmative in casual speech.
That’s actually a spot where hearing “が” versus “か” could really matter. If it’s the former, it probably is the “not,” meaning, since it’s being followed by “but,” and not looking for affirmation. If it’s followed by “か,” it’s almost definitely the affirmative meaning, to the general tone of, “It’s ___, right?” (Though in practice it’s even more strongly affirmative than that most of the time, which makes that a kind of 50/50 translation.) (I also do suspect you’re mostly hearing か, as が strikes me as a less common word for “but” following the casual じゃない, when alternatives like けど exist, whereas じゃないか indeed shows up everywhere, all the time, in anime and manga dialogue.)
There are other non-contextual signals picking up on the “not” vs affirmative difference, but they could be their own thread and you’ll learn them as you continue with grammar. Just be wary, listen carefully, and think about context.