The 'Light Of Apollo' combines an infinite distance line of sight teammate healing beam with the ability to activate a brief but potent regeneration effect.
While its primary function is active, the Light of Apollo changes the HUD by displaying a large white ring around the central point of the player view.
If an allied player is inside the ring, they will be surrounded by a green ring which indicates that they are a viable heal target. By holding LMB, a green semi-transparent beam is emitted from your player model to the target that gradually heals the target in small bursts.
This can be done at any range so long as line of sight is not broken, however the further the target is from the healer, the less healing will be done.
By holding RMB, a bar will appear on the screen and will fill from left to right. Once it is full, regeneration will begin in bursts every second until your health reaches the maximum available.
During regeneration, a pair of large green medical crosses will orbit your player model indicating you are regenerating for the duration of the effect.
When at very low health, keep in mind that being healed by an ally who is using the Light of Apollo takes less time than activating regeneration on yourself.
(guides for this item will be linked here)