21 personnel means that there are two running backs, one tight end, and two wide receivers. 21 personnel is a common grouping for Pro-style offenses. The second running back can act as an extra blocker in the run game, set the strength of the defense, or release as a receiver. The offense remains relatively balanced with two faster receivers split out wide and bigger players inside with the tight end and two running backs. A lot of offenses like to play-action out of this personnel grouping and exploit extra defenders close to the line of scrimmage.
For a quick overview on personnel naming, click here.
With the 49ers using 21 personnel, the Saints match by having seven players in the box to match. That brings them closer to the line of scrimmage and leaves space outside for San Francisco to run play-action.