What it means
Per se means "in itself" or "by itself". It is used to describe something that is true or unlawful on its own terms, without needing to prove anything further. In Australian law you might say conduct is not negligent per se but becomes negligent in particular circumstances. It draws a line between things that are inherently the case and things that depend on the surrounding context.
How it's used
Driving fast isn't negligent per se, but doing so in heavy fog near a school can be.