Click on the center yellow button to Play, then select a level (the more you progress the more levels you can 'jump' to).
Once the game begins, it is your job to keep your little mango monster out of harm's way, while getting all the mangos and other bonus items. To do this, use your left mouse button or your finger (if you have a touch screen) to make your little monster jump. If you are near a wall and you jump, your monster will automatically bounce off of it and climb even higher. You need to time your jumps precisely to get everything and avoid trouble.
Sometimes that means quick reflexes, and sometimes it means solving a puzzle to get to the gate at the end!