Attunement means you must form a bond with them before they can be properly used. Generally, this can be done over a short rest, and afterward, the item can be used as intended. Some items, like the Staff of Power, will specifically state which classes (or even races) will be able to attune to them and use them. Items can only attune to one creature at a time. If another attempts to attune to the item, it will lose its bond to the original creature. Attunement can be lost if the wielder loses the prerequisites to use it, is more than 100 feet away from more than 24 hours, or if they die.