See also: Character Combat
Various attacks, Talents, Feats, and Force Powers can knock a character Prone.
A Prone character takes a -5 penalty on melee attack rolls. Melee attacks made against a Prone character gain a +5 bonus, while ranged attacks against a Prone character take a -5 penalty.
Being Prone may also give a character Total Cover instead of normal Cover (for example, being Prone behind a low wall), subject to the GM's discretion.