Can someone explain how they got g = 9.81m/s^2 ?

[small delta]

@wintry slate g is an acceleration and it represents the net acceleration at which bodies undergo when they are under the force of gravity of the earth and its known to be approx 9.81 or 9.8 m/s2

Its basically a constant

For the most part

For experiments like these where you are limited to a small distance/altitude relative to earth g is considered constant equal to 9.81/9.8 approx..