打盹儿 implies a short, informal nap — often unintentional dozing. For a deliberate nap, 午睡 (wǔshuì, afternoon nap) or 小睡 (xiǎoshuì, short sleep) is more appropriate.
grammar
打 is a versatile verb that combines with many objects: 打电话 (make a call), 打工 (do manual work), 打盹儿 (take a nap), 打喷嚏 (sneeze).
