Pianos are generally tuned to concert pitch and in Equal Temperament.

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Concert pitch,or A = 440 (Hertz, or Cycles per Second) is a reference used by us to keep things more or less standard, though it can vary, both intentionally and unintentionally. Most pianos can be tuned to A = 440 and if they can't, that is, if the pitch of the entire instrument cannot be raised that far, it will be for reasons of age; loose tuning pins, old and rusty strings and so on. Older pianos that are a semitone or more flat are best brought up to pitch gradually, over the course of several tunings.

Pitch is not necessarily an issue unless the piano is to be used to accompany a fixed-pitch instrument, or if a lesson is taking place online and there are discrepancies in pitch either end, or if the player is using an app to learn to play; in my experience, apps will not recognise notes that are more than a quarter tone flat of concert.

 

Equal Temperament is the system of tuning devised to allow the instrument to be played in all keys, equally tuned. This is done by tuning, crucially, thirds (and therefore, also sixths, etc.) with a perceptible beat, so that they are very much not perfectly tuned, (i.e. beatless, pure, intervals), but tolerable, allowing any particular note to fit acceptably into a number of different chords, whereas in most older, "historical" temperaments, thirds and sixths are easier to listen to in some keys, or in some chords, than in others.

Equal Temperament is therefore a compromise that enables the piano to utilise all 24 keys, major and minor, equally.