The normal use of this convention is to check on Aces after the last bid was a natural No-trump.
However lots of club players like to use it at all times to ask, regardless of whether Clubs
have been bid or not.
It is designed to keep you out of a Slam when you are missing two or more Aces.
There different ways of playing it, these are three of them.
You bid 4 ♣ asking Responder to show how many of Aces he/she holds
by bidding progressively upwards
4 ♦ = None
4 ♥ = One
4 ♠ = Two
4NT = Three.
5 ♣ would now ask for Kings and the replies are the same.
Another way (usually referred to as Progressive Gerber) is as above but instead of bidding 5 ♣ to ask for Kings you use the next suit up after the reply- This is okay when you are asking after a No Trump bid but can messy if you use it all the time as the next suit might be Trumps and can be ambiguous.
Another sequence is called Roman Gerber (I think) and modifies the responses to :
4 ♦ = None/Three
4 ♦ = Two of the same colour
4 ♠ = Two of the same rank.
Asking for kings can be either 5 ♣ or the next suit up. See Blackwood
Roman Keycard Blackwood