P/PA is a simple stat that quantifies how many pitches are thrown per plate appearance. It can be used for both hitters and pitchers, although it is more frequently referred to when assessing hitters.
A player can be a very good hitter if his P/PA is below average, but hitters with high P/PA marks can be useful because they tend to wear out opposing pitchers. If a team has a high P/PA mark, the opposing pitcher will often be forced to exit the game earlier than usual.
Similarly, hitters who see many pitches have a greater likelihood of getting the opposing pitcher make a mistake within the plate appearance.