If you use a conventional silver based film then under/over exposure are close i.e. correct exposure is critical.(posted 8610 days ago)However if you use a chromogenic film e.g. Ilford XP2 the lattitude is much larger. For example you can overexpose XP2 by say 3 stops and get excellent prints. If you set your ASA for XP2 at 400 you will get very good pictures but if you wish you can also expose at 50ASA - on the same film in the camera - in order to say get maximum depth of focus on particular shots.
That's why chromogenic films are sometimes called "idiot's film". It's almost impossible to over or under expose. Some very serious and famous photographers use XP2 so don't be put off by its flippant nomenclature.