Attunement

Some Magic Items require a creature to form a bond with them before their magical properties can be used. This bond is called attunement, and certain items have a prerequisite for it. If the prerequisite is a Character Class, then a creature must be a member of that class to attune to the item. If the class is a Spellcasting class, then a monster qualifies if it has spell slots and uses that class's spell list. If the prerequisite is to be a spellcaster of any class, then a creature qualifies if it can cast at least one spell using its traits or features, not using a magic item or the like.
Without becoming attuned to an item that requires attunement, a creature gains only its nonmagical benefits, unless its description states otherwise. For example, a magic Shield that requires attunement provides the benefits of a normal shield to a creature not attuned to it, but none of its magical properties.
Attuning to an item requires a creature to spend one hour focused on only that item while being in physical contact with it. This focus can take the form of weapon practice (for a weapon), meditation (for a wondrous item), or some other appropriate activity. If the creature's focus is interrupted, the attunement attempt fails. Otherwise, at the end of the one hour, the creature gains the ability to use the magical properties of the item. Additionally, when the properties of that item change, such as charges being expended or added, the user is aware of the change.
In order to attune to an item, it must already be identified through magical research including the Identify Spell. An item can be attuned to only one creature at a time, and a creature can be attuned to no more than three magic items at a time. Any attempt to attune to a fourth item fails; the creature must end its attunement to an item first. Additionally, a creature can't attune to more than one copy of an item. For example, a creature can't attune to more than one Ring of Protection at a time.
A creature's attunement to an item ends if the creature no longer satisfies the prerequisites for attunement, if the item has been more than 100 feet away for at least 24 hours, if the creature dies, or if another creature attunes to the item. A creature can also voluntarily end attunement by spending another hour focused on the item, unless the item is cursed.