Excuse my stupid question, but what does social mobility mean? [serious]

Quite simply: The ability to move between social classes.

  Social mobility is the movement of individuals, families,
  households, or other categories of people within or 
  between social strata in a society. It is a change in 
  social status relative to others' social location within a 
  given society.

http://en.wikipedia.org/wiki/Social_mobility