Platforms allow Awesomenauts to accend and decend various parts of the map. To drop through a platform just press Jump and Down and the same time. Some platforms, namely the ones on Ribbit IV and Aiguillon, move up and down.
Some characters and abilities may interect differently with platforms: Derpl's Cat Shot will bounce off of them instead of passing through, Yuri and Ayla (while in Rage mode) can fly freely up and down platforms without the need to imput a special command (just moving through them will do) and Gnaw's Aggressive Acid will cause his Acid Spit to pierce and drip down them.
A platform on Ribbit IV