This is your Hero's repeated cycle of Skills and auto-attacks. This is also used to describe the nature of a Hero's skill in which that skills that are in rotation are neither an Active ability nor a Passive aura, they are automatically used after or before auto-attacks and skills. Being under the effects of Crowd Control, Silence, or using Active Skills may interrupt your rotation and might skip that ability.
For instance, Leandra starts her rotation as follows:
However some Heroes don't always use their skills in the order in which they were learned:
Carrion Beam=> Auto-Attack=> Death Threat=> Auto-Attack=> Auto-Attack=> Carrion Beam...etc.