Surface gravity is the gravitational force felt at the surface of a body (e.g., star, planet). The gravitational force we feel every day is the Earth's surface gravity, which is calculable by considering the Earth's mass at its center and using the Earth's radius as the distance between objects, using the law of gravity.
The surface gravity of a star of a given mass depends upon its radius, e.g., a giant star has a surface far from its central point, and has a lower surface gravity than a smaller star with the same mass.
Surface gravity affects the spectral lines of stars (i.e., gravity-sensitive spectral lines), allowing estimates of the surface gravity to be made from spectrography. Among the mechanisms are pressure broadening (line broadening due to the effect of surface gravity on pressure within the photosphere) and the state of ionization.