It is an acceleration; i.e. it tells you how much your speed changes every second.
It has a value of 9.8 m/s2 (metres per second per second). So, when something falls on the Earth, its speed increases by 9.8 metres per second every second.
It is usually used as the unit of gravity for the surfaces of the other planets.
So, on a planet with a surface gravity of less than 1 g you would feel lighter and on a planet with a surface gravity of more than 1 g you would feel heavier.