Movement Speed is a measure of how fast you can move. Higher movement speed allows you to get from point A to point B quicker, and use less stamina in the process. Your Weight will also affect your movement speed. If you are carrying too much weight, it will slow you down and even stop you completely.
Putting a point into movement speed will increase your speed by 1.5%. On Mobile, it will be increased by 2%. This increases your basic movement speed, as well as your sprinting speed. Points spent in Movement Speed also improve your swimming speed by 1.2%. This benefit is cumulative with any swimming speed bonuses from your Oxygen stat.