Slap is a pizz technique where you get under the string a bit; you pull and release to sound the string. That's followed up by a slap of the pizz hand on the fingerboard (thus the name "slap".)
It's a percussion effect, really. You coordinate into a cohesive whole the sound of the hand slap and the sort-of-violent pizzicato attack on the string. The most simple pattern is using the slap on the offbeat (the ANDs), either straight or swinging. Ya get fancier with multiple slaps: doubles, triples.
You can hear slap in any kind of acoustic music that has DB in it, has no drummer and which gets kinda lively. When I'm playing bluegrass or polka I think of slap as a kind of ersatz oom-pah; fast tempos are OK in that mode. (For some of those kinds of tunes I think I really ought to be playing a tuba or something similar.) For a certain kind of blues, or a great country honky-tonk feel, I like a swung slap; moderate tempos so you can savour it a bit.