In its most basic form, a bid of 4NT, when you have agreed a trump suit, asks partner to show how many aces he holds. In response:
After the response to Blackwood, 5NT asks for kings on a similar scale but with 6NT, not 6♣, showing four kings. Since the 5NT bid commits the side to at least a small slam, it announces that the partnership has all four aces. Most players now use a more sophisticated Blackwood, either Keycard Blackwood (also known as Five Ace Blackwood) or Roman Keycard Blackwood.
You use Blackwood when you think the values for a slam are present but you are worried that the opponents might have too many aces.
You plan to bid 7♠ if partner shows three aces (5♠); settle for 6♠ if partner shows two aces (5♥); or to sign off in 5♠ if, disappointingly, partner shows only one ace (5♦).