A stage in this case is a +50% multiplier to that stat from base value. For example, one Swords Dance raises attack 2 stages, aka +100% from base value, thus one Swords Dance doubles the attack from the beginning. Using it again raises it 2 stages once more, adding 100%, bringing it from 200% to 300%, thus tripling it. The highest it can be raised is 6 stages, or 400% original value. Lowering by stages is more difficult to describe, but, if you imagine the base 100% as 2/2, and raising it to 200% and 300% as 4/2 and 6/2, lowering it two stages brings it from 2/2 to 2/4, or 50% the original value. Intimidate lowers it from 2/2 to 2/3, or 66.repeating 6% the original Atk. value.
Hope this helps!
Thank you that helps a lot.