An allusion, when used in writing, happens if the author mentions something outside the book that the reader wlll be familiar with and be able to relate to. For example, if I was writing a story and I had one of the characters say to another look at another character's invention and say "It's good, but where's the beef?" Of course, you'd have to be old enough to remember that commerical, so maybe another example would be good: One character is mad at another and says "Curse you Perry the Platypus!"
It also works with historical stuff, too, but it would have to be things that a lot of people would recognize, such as: "Bob couldn't enjoy being driven around town in the old, black convertible because his eyes kept scanning for tall buildings or grassy knolls." This is an allusion because it is alluding to the Kennedy assasination.
Get it? It can be tricky, but it's not rocket science : )