What Is Root Motion?
Put simply, Root Motion is the motion of a character that is based off animation from the root bone of the skeleton. Most in-game animations are handled via cycles in which the character's root remains stationary. This cannot always be the case, however, as we can see in the example above. To handle this, we need to take the motion of the root away from the character and apply it to the character's capsule instead. That is the essence of what Root Motion does in UE4.
In order to properly use Root Motion, it is important to note that the Root Bone of your character should be at the origin (0,0,0 with no rotation) as this allows the system to isolate physical movement (capsule) from animated movement (character).
The Root Motion of an animation can be visualized during playback. Simply open up any animation in which the root bone is moving, and in the viewport choose Show > Bones. If you have not selected Enable Root Motion in the animation properties, you will notice a red line that appears as the character's root moves. This illustrates the Root Motion of the animation.
When you enable Root Motion, this red line will disappear. Instead, the character will move in place. This is beacuse, the character's root will no longer move from its original position. In this image, we have enabled Root Motion on the character. Note that we are on the same frame as the image above, but that the character's position has not changed.
So, what does this mean? Now that the system is taking the Root Motion of the character's animation into account, it is then able to reapply that same motion to the character's collision capsule. This means that it can still have the exact same motion that the animator intended, but it can also respond properly to in-game collisions and other physics and gameplay events (which is illustrated in the Root Motion in Action section).