In words, the first equation states that the velocity at time t is equal to the starting velocity plus the acceleration multiplied with the time.
The second formula states that the position of an object equals the starting position, plus the starting velocity multiplied with the time, plus the half of the constant acceleration multiplied with the time squared.
In the last equation it is stated that the velocity squared equals the initial velocity squared plus two times the constant acceleration multiplied with the difference between the current position and the initial position.
In situations with vertical linear motion of objects the acceleration, or deceleration, is mostly caused by gravity. In that case the constant acceleration term in the equations equals minus the gravitational acceleration constant, assuming the positive direction upwards.