In Class: Logos artifact

I brought a lab report from my physics class.
I generally how a mass move when it was dropped down from the resting state, which means the initial velocity is zero.
The lab report also tell me that the method of how to get the motion of the mass including the idea of how to measure some important data. Such us the displacement(distance) between each two dots. It also tells us that why we do not have to time the entire movement since the rbi timer's frequency is a constant, which is 40HZ, so the interval time between the each two dots will be 0.025sec.
I do not really get the idea of how we get the linear regression equation because the proving steps in the experiement is not so convincing.