Root motion means, that the root bone of an animated character is moving, while in non root motion, or in place animation, the root bone does not move. This is contained within the animation sequence.
With in place animations, the locomotion is done by moving the player capsule controlled by the player controller. The animation to be played is determined by the movement of the capsule. If it is moving forward, play a walk animation, if it is even faster moving forward, play a run animation. This for example can lead to sliding feet on ground effect, if animation sequence and character speed do not match.
With Root motion, the overall movement into a direction is already contained in the animation sequence itself. The player capsule itself does not need to be moved at all to make the character move forward. This allows to avoid sliding feet. But it has other side effects for collision - because the capsule, which is used for collision detection, is not moved. So the character mesh moves outside the capsule.
You should carefully read this doc, which really explains it perfectly, as well how to handle it.
Also, you might get a (free) account on mixamo - here you can play around and preview animations and they have some with / without root motion, where you can compare both.