It's an optional field next to your main shipping address on My Profile, labeled "Street Address #2 (Unit, APT, Bldg, etc…)" — for apartment, unit, suite, or building details that don't fit in the main address line (e.g. "Apt 4B" or "Bldg 2, Unit 100"). Fill it in alongside your street address and save — it's stored and shown as a separate line from the main address wherever your delivery address is used.