Moot

Smite is a sumoner spell that you get when u progress

when you level up, and its needed in the jungle to get heal from monster and do pure damage (damage in lol is divided mostly in pure, magic and fisic, pure dmg does not stop against armor or shield).

when your summoner level rise (profile level)

As everyone else has stated, Smite is a summoner spell (like Flash, Heal, Ghost, etc) that you have to select before the game starts while in Champion Select. Smite allows you to buy the jungling item and allows you to smite jungle monsters which deals big damage and heals you.