SLAAC is basically an IPv6 alternative to how DHCP works. With IPv6, you can either use DHCPv6 (ISPs deliver Prefix Delegations and Normal Addresses this way) or SLAAC (How one typically gets an IPv6 address on a LAN or route from a Link-Local address on an ISP).
Hopefully that's clear as mud ;) I would encourage you to go check out IPv6 if that was the intent of your original question. It actually makes more sense after you dive in, and can be pretty neat.
ULAs (Unique Local Address) are one often-overlooked part of which I'm an advocate.
I just replied to myself with an edit to the higher level comment. Sure, I use IPv6 with SLAAC. I'd never needed a separate daemon to handle it, though. I hadn't imagined that OpenBSD would pull that out into its own program, but I'm not at all surprised now that I've heard about it.
ah - gotcha! Yep. OpenBSD is big on the least-privilege principle, Which IMO is why it's pulled out into a separate daemon that only has the permissions and visibility to do what it needs.
Yeah, that seems like a very OpenBSD thing to do, and I mean that positively. It just initially struck me, like, "yay, I no longer have to install a ping daemon!" "A what daemon?"
I could go off and RTFM, but then we wouldn't get to talk about it.