Question

what is time of flight ??

Answer 1

Time of flight describes a variety of methods that measure the time that it takes for an object, particle or acoustic, electromagnetic or other wave to travel a distance through a medium.

Answer 2

Or, in kinematics, it simply means the time during which it is in air. Whatever goes up, must come down according to gravity. After leaving the surface, the time it takes to reach the surface again is known as time of flight.
If an object is launched from the surface with an angle theta to the horizontal, the time of flight is :
\[T=2v \sin \theta/g\]
If you project something vertically upwards, the angle is 90, so:
\[T=2v \sin 90/g=2v/g\]