VIS is Visibility for short. It's important for your map to calculate visibility of stuff you can see or cannot see. It's not always great when a computer does it, so there are a few entities that may further optimize your map when utilizing them.
visibility_hint is a point entity, you can use this to mark an area as "low-resolution" or "high-resolution" for VIS calculations. Low-res would be suitable for large outdoor areas with wide lines of sight. High-res is automatically used for inside places in your map (if your outside environment is marked with low resolution hints)
info_visibility_box is a point entity that is a visibility culling box which can be used to hide objects at runtime and can be toggled on and off by the entity Input/Output system. Mainly used to cull stuff that's within the PVS, but is in an area of the map that the player won't see.
info_cull_triangles is a point entity that is used to prevent overdraw from props/geometry sticking outside the regular map boundaries. You surround an area of the map that has props sticking out (which is usually out of bounds) with an
I hope this cleans up the poor documented nature of these entities. Use them for your own maps when optimization is your priority.