So called because it was invented by a very famous American player Easley Blackwood in the 1930's.
It is designed to keep you out of a Slam when you are missing two or more Aces.
There are other ways but this was probably the first.
This is an Ace asking sequence requiring responder to show how many of Aces he/she holds
You have agreed a suit and one of you decides to check- the asking bid is 4NT and in it's simplest form
you just respond upwards
5 ♣ =None 5 ♦ =One 5 ♥ =Two 5 ♠ =Three and 5NT=Four
Another popular way is to is to play responses on the basis of Pi (03.142)
So 5 ♣ =None or Three- 5 ♦ =1 or 4 and 5 ♥ = Two of the same Colour and 5 ♠ = Two of the same Rank. This can be useful if the asker has a void
Asking usually follows sequences such as 1S- P -4S- P or possibly a strong sequence like 2 ♣ - P - 2 ♠ - P - 4NT which should mean "I think we have enough points and trumps for the Slam but I need to check we aren't missing two Aces"
If after such a sequence Responder now bids 5NT they are confirming that the Partnership holds all the Aces and now is checking on Kings. (might be looking for a Grand Slam)
Now the replies are just progressive
See also Roman Keycard Blackwood