Class M Planet Dead Ahead

I think it is the effect that the gavitaional field of a body has on the light travel close to it. It bends it somewhat, thus causing it to have the same effect as a lens does on light here on earth. By looking for these phenomena scientists can detect the presence of bodies like planets and especially black holes, which have a huge amount of gravitational force. MrMaguro

Here is the Wikipedia definition for what it's worth.
"Gravitational microlensing is an astronomical technique taking advantage of the gravitational lensing effect of a foreground compact mass, such as a planet or star, as it passes very close to the line of sight to a more distant source, such as a star or a quasar. The distinguishing characteristic between microlensing and other cases of gravitational lensing is that the multiple distorted images of the source which the lens may induce are not individually resolvable by optical telescopes. Instead, with microlensing we observe a single apparent image with a brightness given by the combined brightness of the individual images."