Raycasters refer to the raycasting rendering technique where two-dimensional rays are casted from a specific point. (usually player's position)

When a ray hits something, the object it hit is rendered in a way that somewhat looks like this:

As a limitation created by the aformentioned 2D rays, height cannot be properly conveyed without a ton of fuckery.

If you're struggling to understand what I'm saying, here's a (fanmade) map of a few W3D levels. The actual level data is stored in a similar way and the game is programmed in a way more akin to a 2D top-down game. It's just the visuals that are "3D".