A direct heal is a healing effect that provides all of its healing in one shot, as opposed to the periodic healing of heal over time (HoT) effects. Some spells have both direct heal and heal over time components. Direct heals are the healing equivalent of direct damage effects.
Direct heals cause their aggro all at once (when the heal goes off) while HoTs cause a little bit each time they tick. Per point of health healed, direct heals cause the same amount of aggro as HoTs. Direct heals are also sometimes known as "block heals" due to the way they apply their healing in a single block instead of over time.