The disk light shape simulates light from a circular area source (flat disk).

The lights page has more detail about the controls.

Spread

Emits light focused in the direction along the normal. The default Spread value of 1 gives diffuse emission, while lower values focus the light more until it becomes almost a laser-like beam at value 0. Currently fully focused laser beams at value 0 are not supported, there is always a small minimum Spread. Low Spread values can be noisier than the default high Spread, so be careful when using them.

The animation below shows the effect when lowering the Spread value.

_{Spread value lowered from 1 (quad light)}