L5 is one of Google's internal levels. They're putting it there to let people know where they were in Google's hierarchy and they note that it denotes Senior. From what I know, people generally start at 3 right out of school.
To expand a little bit, you usually start at L3 as a fresh graduate, and then are expected to be promoted to L4 within ~2-3 years, and then L5 after another 2-3 years (although it could be shorter). L5 is considered a "terminal" position, in the sense that once you get to that level you're no longer expected to get promoted. (L3 and L4 are, by contrast, more "up or out", where if you don't get promoted after long enough you can get fired.) I actually met an L5 engineer at Google who had been there since 2002. Promotions to L6, L7, etc. start to get exponentially more difficult.