Charge is a magicspell that increases the maximum damage of the God Spells from 18 to 25. This spell requires level 80 magic to use, and you must have a god staff and the corresponding cape to use. P2P mages almost always use this spell before a battle for extra damage in combat.
Charging is one of the best if not the best experience rate in the game. However, constant charging next to a bed is very costly in blood-runes and does not yield any loot or extra benefit. Charging is up to 270k experience per hour.