I believe it is due to the earth's gravity, denoted g, which refers to the acceleration that the earth imparts to objects on or near its surface. In SI units this acceleration is measured in m/s2 (metres per second per second, equivalently written as m·s−2 or N/kg). It has an approximate value of 9.81 m/s2, which means that, ignoring air resistance, the speed of an object falling freely near the Earth's surface increases by about 9.81 metres per second every second. This quantity is informally known as little g (contrasted with G, the gravitational constant, known as big G).
There is a direct relationship between gravitational acceleration and the downwards weight force experienced by objects on Earth. However, other factors such as the rotation of the Earth also contribute to the net acceleration and are usually considered part of Earth's gravity.
The precise strength of the Earth's gravity varies depending on location. The nominal "average" value at the Earth's surface, known as standard gravity is, by definition, 9.80665 m/s2 (32.1740 ft/s2). This quantity is denoted variously as gn, ge (though this sometimes means the normal equatorial value on Earth, 9.78033 m/s2), g0, gee, or simply g (which is also used for the variable local value). The symbol g should not be confused with g, the abbreviation for gram (which is not italicized).