An MP Ring adds an extra point of MP.
If you receive it in an Event (it is commonly given out in events featuring slots or gold decorations), it will automatically be added to your account, and you will receive a notice informing you that you may increase you MP by 1.
If you obtain an MP Ring from a Card Pack, it will be stored in your items page. You will have to manually increase press the 'Use' button in order to use it.
It may be important to note that MP Rings obtained from Card Packs can be gifted to allies and guildmates. However, MP Rings obtained from events cannot be gifted.