Runners are said to be in scoring position when they are on either 2nd or 3rd base, the reason being that they can usually score on any base hit. Runners on 3rd base can score in a greater variety of ways: hit, error, groundout, flyout with less than two outs, bunt, passed ball or wild pitch. Runners at 2nd base can often score even on ground balls that make it past the infield into the outfield.
Smart managers do all that they can to get their baserunners from 1st to 2nd, into scoring position. That is because runners on 2nd base can score a run on even a single, whereas runners on first base cannot score unless a double or triple is produced by the batter. Getting from 1st to 2nd is key, thus managers will often call for a sacrifice bunt to move the runner to 2nd, or order a steal or hit and run. Runners on 1st are also very alert for passed balls and wild pitches, which can enable them to move into scoring position.