In the context of keyboard layouts, Curl is a design principle which aims to deprioritize the centre columns of the "home row" as these feel much less comfortable to many typists, and instead favour the lower-row positions reachable by a relaxed curling of the index fingers while keeping the wrists straight. This principle can be applied during the design of a layout, or can be applied to an existing home-row based layout such as Colemak.
Criticisms of the straight home row in the standard Colemak layout have led to the generation of some alternative layouts that may be inferior in other respects. In the case of Colemak, the two home-row centre column keys are frequently typed letters, D and H. Applying a ‘Curl’ mod provides an improved experience for these letters, without adversely affecting Colemak's many other strenghts.
The ‘Curl’ principle is perfectly exemplified by Colemak Mod-DH. The Workman layout was also developed with a similar idea in mind.
Note: An important observation is that each key moved should be typed with the same finger as the original layout, to avoid any adverse effect on same-finger bigrams (i.e. letter pairs). This means the Curl concept works in harmony with the Angle Mod.