Line Astern is a formation where all planes fly directly behind one another. You need at least two planes for this, but the maneuver is more impressive with more planes (imagine six or seven!)
In this formation, the planes all fly at slightly different altitudes. In fact, these planes cannot fly behind each other at the same altitude because they would end up in the propwash of the plane directly in front. Propwash is air disrupted by a turning propeller, it can render the plane behind it uncontrollable.
You can see the respective differences in altitude well on both pictures above. An additional advantage is that you can see the planes very well, yet you can still see how close they are to each other.
Ready to enjoy something spectacular? Watch a relatively-old-but-recently-edited video of the Victor Romeo Formation in Line Astern, seen from the fourth plane in line.