Bridge auction: 3rd suit tells – 4th suit asks
This Aces on Bridge shows a good example of this.  Look in the &quot;Bid with the Aces&quot; section.
What this describes is an auction where you're exploring NT, and trying to figure out if you're comfortable with coverage in all four suits.
If you have an auction like:




N
E
S
W




1C
-
1H
-


1S
-
1NT
-


2D
-
?
-




2D there would be &quot;asking&quot; for diamond suit coverage.  It wouldn't show coverage, because that's pointless - if N is not worried about diamonds, N would just bid 3NT.
However, in this auction:




N
E
S
W




1D
-
1S
-


1NT
-
2C
-


?







Now you'd be showing a clubs stopper, and looking to see if North had a hearts stopper, since there are two outstanding suits.
All of this assumes fairly basic conventions, as there are other options for conveying this information of course.
It's also worth noting that this is distinct from &quot;fourth suit forcing&quot; - had South bid 2D in the initial auction, that wouldn't necessarily be &quot;asking&quot; for diamonds, it's a generic forcing bid.  As noted in comments, a common response to fourth suit forcing is still in line with this idea - showing the stopper in the fourth suit by bidding NT.