Non-con means non-consensual sex, so basically outright rape; the other person does not want sex and it is forced on them. Dub-con means dubious consent, so it's not clear whether one or both people are consenting. It may be that there's a mutant that causes sexual attraction who makes Charles and Erik get it on for example, or Hank cooks up a love potion in his lab, or much more mundanely Erik forces himself on Raven and she finds herself enjoying it.

In real life, dub-con doesn't really exist. Rape is rape is rape. But in fanfiction and kink dub-con is a handy tool to have violent/out-of-control/interesting etc sex without bringing outright rape into the equation.