Levels in the Quake engine are built out of brushes, a somewhat nonsenical term referring to a set of geometric plane equations that form a convex solid. In lamens terms, any three dimensional shape that does not "cut into" itself at any point.

In the following image, the red line indicates a convex shape while the black line indicates a concave shape.

Jespa, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

Compilation Process

As levels are compiled from the .map format into the .bsp format, the QBSP tool breaks down the brushes into their component faces, and removes any faces that will not be visible from "inside" the level. It also cuts down the other faces to only their visible area. During this process, the original brush shapes are discarded.

For the Quake engine to handle physics and collision, some "hull" shapes must be kept in the final BSP file. Instead of keeping the entire convex brush shape, the QBSP tool "expands" the brushes to simulate collision for 3 distinct "hull sizes". These hull sizes are:

Name	Min	Max	Absolute	Notes
VEC_ORIGIN	0 0 0	0 0 0	0 0 0	This is an infinitesimally small point, used for small objects like grenades.
VEC_HULL1	-16 -16 -24	16 16 32	32 32 56	This is the "small" hull, used for most entities including the player.
VEC_HULL2	-32 -32 -24	32 32 64	64 64 88	This is the "large" hull, primarily used for the Shambler.