I think edge play is pushing at the limits, or edges, of what's safe for you. When people talk about edge play, a lot of times they mean at the edges of what's objectively safe. So, blood play, fire play, breath play, gun play, drowning, possibly suspension - these might all be edge play.

In addition, some people refer to pushing the limits of what's personally safe for you emotionally to be edge play also. Cuckolding or cuckqueaning might fall into this category, extreme humiliation or degradation, realistic incest play... This will vary from person to person. In a way it's by analogy to the idea of playing on the edges of physical safety, but I happen to believe very strongly that non-physical play runs a risk of being really damaging, and therefore can be quite edgy.