The Strehl Ratio is a measurement of Aberration. It has a value between 0 and 1, 1 being "perfect".
Due to Diffraction, a telescope's ability to image detail is limited by the size of its Aperture, each point in the incoming light being aberrated into a spot of light called an Airy Disk, whose size is related to the size of the aperture. The Strehl Ratio quantifies additional aberration by comparing it to this minimal aberration. It is used to characterize optical instruments that image, such as telescopes, and their Adaptive Optics (AO) systems.