Hooboy.
The relevant stuff is:
- Xavier and Cyclops are killed by Jean, who goes batshit crazy in the most stupid way possible, destroying the Phoenix Saga on the way. Wolverine kills Jean, which leaves him deeply traumatized.
- Meanwhile there is some sort of serum which depowers mutants, Rogue takes it in the end. There's some trouble between Rogue and Iceman, as he is interested in Kitty Pryde and of course Rogue doesn't like that.
- Magneto also is depowered by the serum, although not really.
- In a post-credit scene, Xavier seems to have transferred his mind into the body of a brain-dead comatose patient.
Now, they ignore half of that in Days of Future Past in the first place (Xavier looks like Patrick Steward without any explanation as to why, for example), but some parts of it are relevant to understand some of the reactions of characters in the movie. You'll see.
Have fun and see the movie!
