A metastrategy is a strategy aimed not at increasing the odds of survival of one particular character, but on being able to put one of a set of characters in a position of strength for the balance of the game.
Types of metastrategy
Specific metastrategies which are commonly favoured by new players are start scumming (repeatedly quitting and restarting until you get the starting equipment or stats you want) and drinking from every fountain until they either die or receive a wish from a grateful water demon. These are generally frowned upon by experienced players, who feel that novices who play in this way are not developing the important skill of keeping a marginally viable character alive. As many veteran players suggest every game is ascendable, these metastrategies are looked down upon as sacrificing viable characters for a one-time benefit to a specific character.
This will depend on what the player's goal is: if they are trying to maximise the proportion of their games which end in ascension, then by definition metastrategies are a bad idea. If they are instead trying to maximise their ascension rate per unit time, perhaps because they are trying to win the Most Ascensions trophy in a tournament, then metastrategies may be the best way to go.
Metastrategies which are favored by more experienced players include the protection racket (reaching Minetown with as low an experience level as possible so as to be able to buy protection cheaply) and digging for victory (digging down so as to reach the Castle and its wand of wishing as quickly as possible).
In their "pure" forms, these approaches kill off a large number of characters but put those that survive at a tremendous early-game advantage. They can, however, be modified so as to be less dangerous and more useful: it is possible to make a controlled dive to the Castle after accumulating enough armor and other equipment to have a better chance of survival, and it is similarly possible to reach Minetown with low XP gains while not relying on remaining at level 1 or buying all nine points of protection before leveling up normally.
Metastrategies are also sometimes required for particularly difficult voluntary challenges, especially unofficial conducts. For instance, prior to NetHack 3.6.0, zen play absolutely required the character to start scum a blindfold or a towel. Another common scumming target is the ring of slow digestion, which tends to be required in games with particularly large numbers of conducts; foodless play is possible without one, but typically not in combination with many other restrictions as well.