In the movie "Spaceballs", what are the Spaceballs attempting to steal from Planet Druidia?

In the movie "Spaceballs," the Spaceballs attempt to steal the air from Planet Druidia. Their plan is to capture Druidia's Princess Vespa and hold her for ransom, demanding the code to the planetary atmosphere. The Spaceballs, led by Dark Helmet, want to steal the air to replenish their own dwindling supply on their polluted home planet, Spaceball. By stealing the air, they hope to survive and sustain their civilization.