Bidding and playing a slam (12 tricks) is quite rare but earns big bonus points. If you and your partner have a very strong hand or a fit of extraordinary quality, then a slam should be explored.
31+ - 6C, 6D, 6H or 6S
33+ - 6NT
35+ - 7C, 7D, 7H or 7S
37+ - 7NT
By checking how many Aces and Kings you hold between both hands, you can bid a slam with confidence.
The convention we use to count Aces and Kings is called Blackwood (4NT).
The Gerber (4C) convention, like Blackwood is a way of determining the number of Aces when looking for a slam. Gerber is only used when the partnership has agreed that the contract will be in NT.
Roman Key Card (4NT) is a commonly used variant of Blackwood. The king of trumps is counted as an Ace (so there are 5 aces). Trumps are defined as the last suit bid.