Pumpy can only jump and push some objects. Each level contains some doors that need to be opened by pushing particular blocks on their corresponding sensor. Here is an exemple:
On this screen (the first part of the first level), you can see a sensor cell, a key block and a door. In order to open the door, all the key blocks must be pushed over all the sensor cells of the level. In this case, there is only one sensor, but some tricky levels can hide many sensor cells.
Moreover, you must take care at the color of the sensor cells, the key blocks and the door. Purple doors are opened by purple key blocks placed over purple sensor cells, and so on.
The game ends when you exit from the last level.