In this video the author explains the various concepts like speed, velocity, free fall speed etc., Speed tells you how fast you are going, i.e. it is distance divided by time taken to travel it. Velocity is nothing but speed along with direction. Now she tells that acceleration is used to measure the rate of change in velocity. So she says that acceleration is difference in velocity divided by the time. She also explains that there can be acceleration with out change is speed as there can be a change is direction. She further explains about the free fall speed saying that the gravity of the earth is 9.8 m/sec*sec which means that a body freely falling increases its speed by 9.8 for each second. The author clearly explains these concepts clearly with illustrations. To understand these basic concepts of physics watch and learn from this video.