Mask

In GM, normally an object's sprite is used for collision detection, but if you don't want the sprite to be used for collision detection, you can use a mask. A mask is basically a sprite which isn't shown in game, but is used to detect for collisions instead of the objects sprite. This can be very useful when the object has multiple sprites which are animated. They are in fact used as the solid part in objects. Let's say that I have a platform game with animated sprites that keep getting caught on the walls. I could use a square sprite as a mask, and the problem would be fixed.