The Plus key is used for making things better than before. Adding + or even ++ after a word expands it into something fabulous. N00bs are advised not to overuse the plus key as it makes things look like a cemetary.
The plus key is also used for something called arithmetic. As such, it is then part of a complicated subject beyond the scope of this article, making our collective heads hurt. A similar but much better use is for putting combinations of things together, like Big Bird + Kermit. This allows you to get a good start in writing slash fanfic.