a 22m drop out comes after the ball has gone dead from the oposing team, or if the defending team grounds the ball when retriving the ball inside the goal area.
Then play is to resume, from a drop kick from the 22m line. This can be done straight away but the only offside rules are that all people in the kicking team has to be behind the kicker, and the defending team cannot go over the 22m line. so effectivly the 22m is the offline line.
There is no minimum distance the ball has to travel. it just has to be kicked over that 22m line.
hope this explains it, but i am sure someone else out there can say it better.