In plain language
The site must work in both portrait and landscape unless one orientation is truly essential.
Who it affects
People whose devices are mounted or who cannot rotate a display freely.
How to check
Rotate the device/emulator both ways: everything must remain usable, with no 'please rotate' blocker. Worked example — A photo-sharing app's upload screen: rotate to the 'wrong' orientation and inspect: CSS media queries that hide or break all content in one orientation (e.g., @media(orientation:landscape){display:none}) fail.
Common misconception
Teams assume this only applies to phones. It also covers any device that can be mounted or fixed in place, like a tablet on a wheelchair, where the user genuinely cannot rotate it. Real example: on a real-estate marketplace's listing page, the app locks itself to portrait or landscape — users with a mounted device (e.g., on a wheelchair) can't rotate and can't use it.