Player created sets of units which can be selected instantly. Often players will create different groups of units based on their function. One preset group might contain all fast units (scouts), one preset might be artillery and their guards, another might be all melee units, etc.
A player can create up to 10 presets which can be invoked in game by pressing the command + a number key (1-0).
Presets are useful because it is quicker to press 2 keys to select a group of mixed units than it is to try to select units in the heat of battle.