HoT is an acronym that means "Heal over Time".
It refers to spells, such as Renew or Rejuvenation that have a quick or instant cast and apply periodic healing over a set amount of time. Though spells like Regrowth have a short cast time and heal for a larger amount initially, it's still considered a HoT because after the initial heal it applies a certain amount of healing over a set amount of time.
However channeled spells such as Tranquility are not referred to as HoTs.
See also DoT