A lens throat is the inner diameter of a camera lens mount. It is the width of the opening, measured between two non-obstructed points, i.e., not including the mounting tabs on a bayonet mount.
Canon stated, "Throat size is crucial for determining the angle of incidence and provides us with a more realistic picture of the mount's capabilities."
In an interchangeable lens camera, the combination of the throat diameter and the flange distance affect the angle of incidence. A larger throat and a smaller flange distance make for a larger angle of incidence, allowing for more flexibility in lens design, without requiring retrofocus designs for wider angle lenses.
The elements of a lens can have wider diameters with a larger throat diameter, meaning that surface curvature can be less drastic. And with a well-designed lens with lessened surface curvature you get less chromatic aberration.
See also
Camera Geometrical optics Lens adapter
References