A hitter plays every day and hits once every nine spots in his team’s lineup.
A pitcher plays every fifth day and faces all nine spots in the other team’s lineup.
For example:
Albert Pujols has had 21 plate appearances in the last five games. For the season, he averages 21.3 PAs per five games.
Tim Lincecum had 29 plate appearances against him in his most recent game. For the season, he averages 28.6 PAs against per game.
Joe Mauer has had 20 plate appearances in the last five days. For the season, he averages 22.1 PAs per five games.
Zack Greinke had 29 plate appearances against him in his most recent game. For the season, he averages 28.2 PAs against per game.