FIMming is similar to topping, in that you are chopping the top off your plant. The difference with fimming is that you leave like 10% of the growth tip, rather than removing it completely. The effect of this is to produce several growth tips in place of one (usually 4 tops, but sometimes up to 8, rather than the 2 produced by topping).
Here is a good thread about FIMming, complete with pics. There are some good photos in the first post, and a very descriptive diagram on the second page (edit: which Boneman already posted, lol).
As a bit of trivia, FIM is short for "fuck, I missed", since it looks like the result of a botched topping.