It basically says that in a circle, if two chords $ AB$ and $ CD$ intersect at a point $ M$, then $ AM \cdot MB = CM \cdot MD$.

Keep in mind that $ M$ can be outside the circle (we extend the chords until they intersect) and either or both of the chords can be replaced by tangents (in which case $ A = B$ or $ C = D$). The numerical value of $ AM \cdot MB$ is called the power of $ M$ with respect to the circle. By the way, this would've been very easy to look up for yourself.

Oh, I see (I think). Thanks! Oops. Posted in High School Math. Silly me.

Also learn to use the Wiki: Power of a Point.