Ah! Ok... if you look up the word in a dictionary you'd get this:
spiffy A adjective 1 dapper, dashing, jaunty, natty, raffish, rakish, smart, spiffy, snappy, spruce
marked by smartness in dress and manners; "a dapper young man"; "a jaunty red hat"That's what the word
actually means.
They way we use the it
here though is slightly different. We use it as a word for something that we like and enjoy. Something cool, good and/or exciting.
Like so: "Ooooo! That's spiffy!" or "I had a spiffy time!".

<- Some spiffy smileys.
It's up to you how you want to use it really. In short; it's forum slang.