The surface reflectance product is the input for product generation for several of the land products: Vegetation Indices (VIs), BRDF, Thermal Anomalies, Landcover, Snow/Ice Cover, and LAI/Fpar. It is an estimate of the surface spectral reflectance for each band as it would have been measured at ground level if there were no atmospheric scattering or absorption.
See links below to the Product Description pages posted at the EDC DAAC (product details, data access links, user guides and more....)