Going off of what ArtOfWarfare said, this is a very common but simple technique used in many side scrollers (Jetpack Joyride, Funrun, etc). Basically you might have 2-3 "layers", the frontmost being the important game objects that the player can actually interact with, enemies, power ups, items, and such. The middle layer can be objects in the near background, like trees, rocks, buildings or whatever. And the last layer will usually be the setting of the game, like mountains, forest, cave, etc. And to make it all come together you simply make it so that the lower layers move slower than the top layers. It gives the illusion of traveling.