Each roll of film is manufactured at a certain ISO rating, or sensitivity to light. The higher a film’s ISO rating, the more sensitive it is to light and the less light required to properly expose a frame.
When you rate your film at a lower ISO than it was manufactured at, you are exposing your frames as if your film is less light sensitive than it is in actuality. Technically, this means you’ll be overexposing your roll. Different film stocks look better exposed at different ISO ratings, so it’s helpful to run a few test rolls of difference stocks at different exposures to see what look you prefer. For example, I find Fuji 400H can be overexposed more than Kodak Portra and still look gorgeous.