Being added as an authorized user means someone adds you to their existing credit card. The card's history — its age, limit, and on-time record — can then appear on your report, often within a billing cycle or two, without you ever using the card. It can be a real help.
But the most important thing to understand is this: the benefit is borrowed, not owned. It depends entirely on the other person keeping that account open, paid on time, and at a low balance — and on their card issuer actually reporting authorized users to the bureaus (not all do, and not always to all three). If they're removed, miss a payment, or run the balance up, the effect can reverse — sometimes at the worst possible moment, like right before you apply for something. So treat it as a supplement to your own accounts, never your only plan, and never count on it for a specific approval.
A good authorized-user account is old, spotless, and low-balance, and belongs to someone you trust who'll give you a heads-up before removing you. Confirm their issuer reports authorized users to all three bureaus before you count on it; whether and how any negative activity shows up on your file can vary by issuer and bureau, so it isn't fully predictable.
One hard rule: never pay a stranger or a "tradeline" service to add you to their account. That's a different, risky thing entirely — we cover why in the Proceed With Caution area.