centos - What is Slab, SReclaimable and SUnreclaim? What does Slab solve and how does it behave under load?

I have a server with 32 gigabytes of ram, with htop reporting 29 gigs used, with 4 gigs of swap in use as well.

cat /proc/meminfo returns around 13 gigabytes of memory listed as SReclaiable.

What is Slab? When was it added? What does it mean it is reclaimable, when does this happen? Why am I still stepping onto swap space if there's 13 gigs free?

I've found the slab allocator docs to be WAY too Unixy for someone just looking for an answer, so I'm hoping someone could provide a reasonable rundown of what it is and how it behaves.

By the way, it's a CentOS 3.10.0-123.13.2.el7.x86_64 in this case.

PS. I realize the question may be imperfect in many ways, but I'd love for a practical guide to slab surface somewhere. Maybe we could start here?