The clue here is really in the name, a healers job in your team is to keep the rest of your team alive – either through team healing, direct healing or heal over time abilities (HoT)
What is an AOE heal vs a Single Target heal?
An AOE heal is an ability that affects the whole of your team, for example, Bad-el Kazar’s A1 ability will heal every single member of your team when he attacks, abilities like this are really helpful to keep your whole team topped up during an encounter helping you to clear said content.
A single target heal will only heal 1 team member, targeted on a champion of your choice (if on manual) or based on specific conditions, for example some abilities my heal the lowest member of your team. Note, when running on auto your healers will usually prioritize the team member with the lowest HP if it’s a targeted ability.
What is a HoT?
A HoT stands for heal over time, which as it implies, heals the members of your team over time, in many games this will be a ticking buff that will tick every x seconds, healing you every time.
In Raid, HoTs come in the form of the Continuous heal buff, which heals you by either 7.5% or 15% of your max HP every time you take a turn, for the remainder of their duration. Check here for Champions who have the ability to place such buffs.