So I just learned what this is, I was curious too:
You know when you run up against a wall in a video game, and then instead of just sticking into the wall, or bouncing off the wall, you slide one way or the other?
This is what the slide function does
So you would use it if you were writing some code that involved some body, maybe, but a vector, at least, coming into contact with a plane (assuming its in motion), and then you would say, when it hits the wall, slide along the direction of the wall, which off the top of my head seems like a vector perpendicular to the normal of the plane.
I can see this being used in kinematic body, where kinematic body physics need to be written by the coder as constraints instead of just generated dynamically like rigid body.
Anyway, I watched this talk and learned that:
I think there is an example of slide in the kinematic body demo
