There is a brilliant essay by Mark Lynn Stoddard at The Black Blog of Nameless Cults:
The three acts defined by Campbell are Departure, Initiation and Return.
In the Departure the hero receives the call to adventure. They may or may not resist the call and usually encounter a mentor to advise and guide them.
During the Initiation the hero passes into the unknown, a cave or some type of special world which they must navigate through. This is the place where the hero must undergo trials and overcome obstacles. There may be helpers along the way. Eventually the hero reaches the “innermost cave” and goes through an ordeal concluding with the final “boss fight” to gain the reward.
Return sees the hero coming back to the ordinary world with the reward. The reward may be a treasure, a magical elixir, wisdom or even spiritual power over both worlds.