When we have a fit for partner’s suit and are considering how vigorously to support partner, we notice our Cover Cards (that is, our ability to eliminate (“cover”) losers in partner’s hand). Cover Cards most frequently come from top honors or from the ability to ruff.
Top honors in the supporting hand are presumed to be Cover Cards in these situations: The Ace (unless there is some reason to think that partner is void); the King (unless there is some reason to think that partner has a singleton); and the Queen (in any suit in which partner is expected to have at least three cards.
Regarding ability to ruff: Voids, singletons, and doubletons in the supporting hand are presumed to provide Cover Cards whenever you can expect to ruff a loser from the long trump hand. For example, a singleton in the supporting hand opposite Axx in the long trump hand is presumed to provide two Cover Cards by ruffing, so long as the supporting hand has a sufficient number of trumps.
Sometimes, extra trump length in the supporting hand provides a cover card; the most common example is xxxx where the long trump hand has AKxxxx (no trump loser so long as the opposing trumps split 2-1 which is almost 80% according to the textbooks).
Here is a simple example for Losing Trick Count and Cover Cards (spades are trumps):
Declarer (the long trump hand) has a Losing Trick Count of seven (one in spades, two in hearts, one in diamonds, three in clubs). Dummy (the supporting hand) has three Cover Cards (the King of hearts, the Ace of clubs, and a ruff in hearts), so game in spades would have poor chances (seven losers, minus three cover cards = four losers = nine tricks not ten). The long trump hand is a good example of a hand that would have reasonable prospects for game opposite a limit raise (4+ spades, 10-12 support points), possibility for game opposite a mixed raise (4+ spades, 7-9 support points), and poor prospects for game opposite a single raise.