EMP is "electro magnetic pulse" It's kind of like one of those star wars-star trek weapons that they always talk about that will knock out communications or weapons, except that it really exists. on your bike it is a very small scale, but its enough to screw up your speedo.
Basically you are getting interference from the magneto on the motor. An emp blocker (or suppressor) will shield the cable on your speedo from the electro magnetic pulses.
If you want a better explanation you will have to visit this new webpage that I designed (been working on it for years...) Google
it knows just about everything