These tokens are used specifically with the Apothecary class. When a player chooses the Apothecary class during Hero Setup, he places the elixir tokens in a supply near his play area.
Each time a player is instructed to gain one elixir token, he takes one elixir token from the supply and places the token on his Hero sheet. Each time a player discards one elixir token, he removes one elixir token from his Hero sheet and places it back in the supply. Elixir tokens remain on Hero sheets between encounters but are discarded at the end of a quest. The number of elixir tokens a hero may have on his Hero sheet is limited only by the supply.
Any hero with an elixir token has the option of discarding it at any point during his turn to recover damage. This ability is printed on the “Brew Elixir” Apothecary Class card as a reminder. When a hero discards an elixir token, he rolls one red power die and recovers damage equal to the rolled on the die. A knocked out hero cannot discard an elixir token during his turn.
Heroes may trade elixir tokens following the trading items rules on page 15 of the Descent: Journeys in the Dark Second Edition rulebook.