You learn the Fire Bolt cantrip. It counts as a warlock cantrip for you, but it doesn't count against your number of cantrips known. When you hit a creature with Fire Bolt, the creature needs to make a charisma saving throw. On a failed save, the creature is ignited and takes additional fire damage equal to your spellcasting ability modifier. It can retry the save at the start of each of its turns; on a failed save, it will take the additional fire damage again.