As far as I remember from the HL source code, "agibs" are alien gibs (from the Xen monsters and the like) and "hgibs" are human gibs (from players, scientists, grunts, etc). Blood works the same. Human blood is red, alien blood is that weird light-green color.
Those types of classifications are basic to HL, so they're in ESF too. Players are regarded human, and env_models and whatnot are regarded as aliens (they bleed the weird alien blood sometimes).
Without doing any research, I'd venture that the blood svars toggle the spawning of blood, and the gib svars toggle "gibbage" on death. Violence settings can be set on the client, but if they're set on the server (I didn't know they could be, but why not?) then they'd probably just override the client settings. If that's right, then you could have gibs and blood enabled on your client, but join a server with them disabled, and you wouldn't see them.
So yeah... setting them to 1 would enable them, which is the default. Nothing you have to worry about.
