Player and GM roll d20, under or equal the number is a success, 1 is a crit, 20 is a fumble, other is failure. You compare the result Grug crits his Mighty Axe while the goblin fails his Shifty Nature. This gives you a matrix of results but most often you just care about success vs. failure (but it could be the difference between killing the BBEG or just being able to rescue the princess while he is able to escape).