You can modify a gradable adjective to show various degrees, by either strengthening or weakening- something can be really hot, slightly hot, or not so hot.
Non-gradable adjectives are for extremes, absolutes or permanently defined states. You can only modify non-gradable adjectives to intensify. Take "fantastic." So something can be utterly fantastic (speaks to intensity) but you can't say something is "slightly" fantastic. Or think of "dead." You can't be partially dead, but you can be stone cold dead.