In this classic level you need to push most of the boxes through the goal area since you can only fill it from the right side.
If you do not move any boxes to the goal area in the beginning the first five boxes to move is more or less given. The sixth? Be sure you only give it one push.
The trick is to store some of the boxes in the goal area and move most of the rest through.
You get a very clear idea from the next snapshot: