The phrase "land surface" refers to the topmost layer of the Earth's land, which includes soil, rocks, and vegetation. It is the part of the planet that we see and walk on. Full definition
Scientists combine measurements of land surface temperatures and sea surface temperatures to calculate the global average temperature. (climatehotmap.org)
However it should be noted that satellite - based estimates of temperature are a less appropriate measure of land surface temperature than those derived from ground - based stations. (bom.gov.au)
A later study using a global land surface model found similar results: cool roofs could offset the emissions of roughly 300 million cars for 20 years. (newscenter.lbl.gov)