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.
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.
Metastrategies which are favoured by some experienced players are 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).
What these strategies have in common is that, in their pure forms, they 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. For example, it is possible to make a controlled dive to the Castle after accumulating enough armour and other equipment to have a better chance of survival, or to try to reach Minetown with low XP but not insist on being level 1 or on buying all nine points of protection before beginning to level up normally.
As many veteran players suggest every game is ascendable, metastrategies are looked down upon as sacrificing viable characters for a one-time benefit to a specific character. This may 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.