When you define an object in GameMaker Studio 2 you can assign it a mask to be used for collisions rather than the one that corresponds to the defined sprite. This variable can be used to find the sprite_index of that mask (or it will return -1 if no sprite has been assigned) or to set the mask for an instance to the chosen sprite. Setting the mask index means that you can have, for example, a sprite for the instance with an irregular shape, yet give it a circular collision mask that is gotten from a different sprite.
mask_index = spr_Round;
The above code sets the mask of the instance to that of the sprite "spr_Round".