Your song of rest of become so potent that you can bring creature who hear it to near full strength. When you use your song of rest feature, you can expend a bardic inspiration. When you do, you choose a creature who was effected by your song of rest. That creature regains additional hit points equal to the roll of your bardic inspiration die, and also either regains an expended spell slot or regains use of an ability that normally recharges after a long rest. If you choose a spell slot, the regained spell slot must be of a level equal or lower than the roll  of the bardic inspiration die.

You can do this once, regaining use after a long rest.