You get foggy film.
The effect is much worse when undeveloped film is packed in checked luggage rather than carry on, as the X-ray intensity is much higher. The more times the film goes through the X-ray the more faded/washed out the images will be. If you do need to travel with undeveloped film then be sure to take it in your carry on luggage and ask the airport staff to exclude it from the X-ray. It’s best if you have it in a separate clear plastic bag and ask as you hand over your belongings for scanning.
I completely forgot to do this with my latest films and while one roll was ruined, another had a distinct vintage feel to it that I actually really liked. Here are some of the shots of Cornwall from that roll.