Air bubbles are spheres of air found underwater. They are produced by Carmen the Whale in Whale Bay, and they replace the Blue Lums in Rayman Revolution. When Rayman collects an air bubble, his air gauge fills and he is able to swim for longer resurfacing.
In Rayman 2 and Rayman Revolution
In the original versions of Rayman 2, air bubbles are only seen in Whale Bay, being produced by Carmen the Whale. Blue Lums are the main source for Rayman to fill up his air gauge. Rayman needs them when he swims underwater, as he will die if he doesn't refuel with air bubbles. Piranhas also seem to feed on air bubbles, but it's most likely because they are hungry for them.
In Rayman Revolution however, air bubbles are also seen in the Fairy Glade, the Sanctuary of Water and Ice and again in Whale Bay. They can appear as small little bubbles forming vertically or the usual single large bubble that Carmen gives out. Air bubbles are also seen (albeit briefly) in the Minisaurus Plain, where Rayman first learns to swim underwater.