When working with animation, it is very useful to see the trajectories of objects. From them, you can immediately estimate the speed and position of the object in all the desired frames.
By default, if several objects are selected at once, you will only see the trajectories of the highest objects in the hierarchy.
By default, you will see trajectories for those objects and frames that are currently selected. If objects or frames are not selected, you will not see any trajectories in the viewport.
Often, however, you want to see object trajectories even if they are not currently selected.
Sometimes it is necessary to add trajectories of new objects to already anchored ones.
Sometimes it is necessary to temporarily hide the trajectory display.
You can also see the position of objects in other frames as silhouettes.
The listed above options of working with trajectories are enough to start making animations and tracking the movement of the character. However, this is far from all that they are capable of! You can learn more about the trajectories from the video: