I'll give you the really basic version of what happened and then you should go off and read the full version in the link above.
So Mr. Gaiman was doing a book signing and Jason decided he wanted to ask his girlfriend, Maui, to marry him. Jason's brilliant plan (no sarcasm, this is a brilliant plan) was to ask Mr. Gaiman to sign his girlfriend's book by putting Jason's marriage proposal in the message. Now, Mr. Gaiman could have said "well that's not for me", but no, he agreed (score one point for Mr. Gaiman). He wrote the note and handed the book to Maui and, she being such a fan, didn't read it. Mr. Gaiman said "you have to read what I wrote", she glanced at it and got all excited, even though she hadn't read it. So Mr. Gaiman insisted, over and over, until finally she broke from her moment of starstruck paralysis and realized what was going on. In comes Jason, kneeling with ring in hand.
That's the short version and it doesn't have the lovely pictures of the actual story. But let me just say that this is by far one of the coolest things a writer could have done and what an amazing way to propose to the woman you love. Congrats to Jason and Maui (I don't know you personally, but congrats anyway) and I hope you two have a great life together!
What an amazing way to propose!