This function defines a circle shape for your fixture with a radius defined by the argument "rad".
|fixture||the index of the fixture|
|rad||radius of the circle|
sprite_get_width(spr_Ball) / 2);
The code above will apply a circle shape to the fixture indexed in the variable "fix_Ball" with a radius the same as that of the width of the sprite "spr_Ball" divided by 2.