This is an animation tool for tracking arcs on objects. It allows you to reposition where the trail is tracked from (for example, you could add a trail to the head, but reposition the trail to track the arc of the nose). The trails will update while you adjust animations. You can adjust the range of frames that the trail is drawn across (useful for polishing small chunks of an animation at a time) and can easily edit the thickness of the trail.
The spacing marker option creates visual markers identifying the position of the object on a frame by frame basis, even if the object being tracked is inheriting motion from another object (for instance, tracking a wrist that inherits motion from the body and upper arm).
Trails can be easily managed or removed using the built in trail list. You can double click an object listed in the trail list to select that object. Remove trails by highlighting objects in the trail list, or selecting them in the Maya viewport, then pressing 'Remove Trails'. Pressing 'Remove Trails' with no objects selected will remove all trails currently in the scene. Use the 'Edit Trails' to apply the settings in the UI to the selected trails, or to force a trail to update.