Stacking is when one or more effects pile on to each other. For example, a Wookiee Scout has Momentum. He moves and attacks another character, giving him +4 attack and +10 damage. Tarfful was within 6 squares, granting him another +4 attack. The wookiee scout would have +8 attack because these effects stacked.
However, unless otherwise specified, no effect produced by a Force power, special ability, commander effect or cover stacks with itself. For example, the Battle Droid Officer can give nearby Droid allies a +4 bonus to Attack but two Battle Droid Officers can't give Droid allies a +8 bonus.