This setting determines whether origin motion keeps the rings together, or spreads them out. Trail ranges from 0 (none) to 100 (maximum). When trail is zero, the rings stay together; as trail increases, rings further from the origin lag behind, creating a slinky effect. When trail is at maximum, rings don't move from their birth positions. In short, trail controls how long it takes for the rings to recenter themselves around a new origin.
Trail is essential for simulating elastic motion, e.g. jellyfish, worms, etc. The best results are achieved by combining trail with damping, and limiting the number of rings, via the rings setting. Random origin motion works well, because the pauses give the rings a chance to catch up periodically, instead of remaining spread out all the time. If the rings are filled, it may be more effective to draw them in convex mode, so that the innermost rings are always visible.
Trail can be changed via the Master dialog, MIDI, or by moving the mouse wheel while holding down the Shift key.