Festoons or a festoon (under one eye) is a condition where the tissue of the lower eyelids gets swollen.  This may be due to conditions like allergies and may also be due to a genetic predisposition to the chronic retention of fluid in the lower lids.  Festoons are very different from lower eyelid bags, which is simply loss of the tissue volume just below the normal fat around the eye causing the lower eyelid to look like it has puffy bags underneath the eyes.  This volume loss occurs as we age and contributes to the tired look that can develop in the lower eyelid area.
If you have true festoons, there is no real treatment for the underlying cause although tightening the skin in this area can camouflage the problem.  Injections with any filler will not help but only serve to make things worse if indeed you have this problem.