The Single Look Complex SAR format is a single look product of the focused signal. It means that the azimuth compression has been carried out using the full azimuth bandwidth and therefore contains the highest azimuth spatial resolution and at the same time, it suffers from maximum speckle. The data are in the radar geometry, i.e., in slant range coordinates, not projected onto any reference surface. Each pixel of the SLC product is a complex number. (i.e., has a real and imaginary component) that represents the amplitude and phase.
From the Single Look Complex (SLC) product the Multi-look Detected/Multi-looke (MLD/MLI) can be generated. It is produced by multi-looking, i.e., averaging, over range and/or azimuth resolution cells.
Precision Images (PRI) are the Multi-look Detected/Multi-looked Intensity (MLD/MLI) images that have been resampled into square pixels, rotated to account for the view direction of the instrument and warped by some predefined operation that the projected image pixels are georeferenced onto a specified geographical coordinate system.
Before performing multi-looking, the Single Look Complex (SLC) slant-range geometry is projected onto ground. This kind of product, i.e., in ground range geometry, is known as a Ground Range Detected (GRD), e.g., product of the Sentinel-1 mission.