Onboarding is the deliberate sequence of in-product moments designed to reach the aha moment as quickly as possible. Effective onboarding combines instruction (explaining what's possible), action (forcing the user to do the thing), and reward (showing the outcome of doing it).
The choice between minimal onboarding (let them explore) and guided onboarding (walkthroughs, checklists) depends on product complexity. Activation rate is the standard metric; retention curves are the longer-term test.