What does 'to fall behind' mean?

Per Answers.com, to retreat, hang (back) or fall (behind) in movement, progress, development, etc.

Synonyms: http://www.answers.com/topic/fall-back-and-forward, http://www.answers.com/topic/lose, http://www.answers.com/topic/drop-off-wordnet, http://www.answers.com/topic/recede, http://www.answers.com/topic/lag, http://www.answers.com/topic/dawdle, http://www.answers.com/topic/fall-back-and-forward