The term may be used to compare different minions of the same mana cost (" lacks the powerful abilities of other 7-drops"), or when discussing specific mana constraints ("I could play two 3-drops or one of my 6-drops this turn"). "Drop" is often used to refer specifically to a minion that can be played effectively at that mana total without special strategic consideration ("I'm mulliganing my to try to get a real 3-drop like , since I don't want to play the Owl on turn 3").
For more information on minions in general, see Minion.
In an extremely rough sense, the "power" of a card is proportional to its mana cost. Thus, two 3-drops might be expected to be "equivalent", and trade about evenly with each other if put into combat. A 4-drop would be expected to "beat" a 3- or especially a 2-drop, either killing the lower drop without dying, or trading with it while accomplishing some secondary task, such as drawing a card.
It should be heavily emphasized that this is a very general view of card power. The board situation, minion statistics and abilities, spells and Hero Powers, and overall quality of each card will frequently upend this "rule of thumb". Thoughtful decision-making is therefore far more important than playing the "biggest drops". However, it is still a useful perspective in deck construction, and other contexts in which the specific gameplay situation is unknown.
In a deck construction and game planning context, the number in front of "drop" usually refers to the earliest Mana Crystal total at which the minion could be played to good strategic effect. For example, is a 2-drop because it is usually a reasonable play on turn 2 (or turn 1 with ), whether or not the opponent has a minion. On the other hand, may not qualify as a "real" 2-drop because players usually save its buff effect for a specific situation instead of playing it on turn 2.
This does not mean that such non-drop minions are "bad" - indeed, cards with special abilities can often swing the course of a game. It simply means that they are not cards players plan on playing "on turn X" as part of an overall strategy for the match. Depending on special abilities, some cards may be "conditional" drops of some amount, meaning that they would be good plays on that turn under some circumstances, but not all. For example, a might be considered a "possible 5-drop" since it is an excellent play on 5 mana if the opponent has a 1- or 2-attack minion, but may be wasted if played against other minions.
In most cases, the number of the drop identifies the first turn on which the minion can be played, since this is usually the same as the mana available on that turn. Thus, the second player's 2-drop often faces the first player's 2-drop at the end of turn 2, their 3-drop faces the opponent's 3-drop, and so on.
Some cards adjust the tempo of the game by modifying the mana available, allowing or forcing a player to play a higher- or lower-value drop than would normally be possible on that turn. For instance, might allow playing a 4-drop on turn 3, while enables a druid to play drops one higher than normal on all subsequent turns. Because a drop, on average, is expected to "beat" a drop of lesser value, this may give the player a strategic advantage. Inversely, a turn 2 forces a shaman to make do with a 2-drop on turn 3 due to Overload, while reduces the maximum drop playable on each turn that follows. Players must judge whether the immediate advantage of such cards is worth the disadvantage of being restricted to inferior drops afterwards.
Importance to strategy
The numbers of drops at each mana cost are a primary factor in the mana curve of many decks, particularly Arena decks. If a player can't spend all the mana on a given turn on a drop or combination of drops, mana or card value may be wasted, shifting the tempo or card advantage unfavorably. Players are therefore dependent on drawing early drops in the first few turns, which is only likely if enough such minions are in the deck. Searching for low-mana drops in the mulligan is also usually advised. It is possible that situational spells or minions can be played to their full potential on those turns, but from a planning standpoint, players cannot count on an ideal situation, and would be wise to plan their mana curve accordingly.
Having an exactly matching drop becomes less important in later turns when increased resources give players a wider variety of options. For instance, a 4-drop and a 5-drop, or a 3-drop and a 6-mana spell, is a fine play on turn 9, and those cards have the advantage of being playable on earlier turns if desired. Actual 9-drops are comparatively rare and harder to play. This logic can be extended as far as playing 2-drops and 1-drops on turn 3, although decks not specifically aiming for aggro tend to end up somewhere in the middle.
The shape of the mana curve is also highly dependent on the specific deck in constructed play. Different strategies include a variety of tempo goals. Control decks may contain combos capable of overcoming early tempo deficits and therefore average much higher drops, while aggro decks typically contain only very low drops. In both cases, the carefully tuned synergy of the deck allows it to deviate from the natural mana curve.