The phrase is usually treated as a technical instruction about breakpoints. Its actual value is editorial.

The constraint does the work

On a wide screen you can place everything and defer the argument about priority. On a narrow one you cannot, so the team has to agree what comes first, second and third. That agreement is the useful artefact, and once you have it the wide layout usually improves too — because it now has a hierarchy rather than an arrangement.

Where it is misapplied

• Hiding content on small screens instead of prioritising it. If it does not matter on a phone, ask whether it matters at all.
• Treating a hamburger menu as a solution. It is a place to put things you have not prioritised, and discoverability drops measurably once something goes in.
• Designing the mobile layout as a narrower desktop rather than a different reading order.