I'll not change a cottage in possession for a kingdom in reversion.

The speaker values a humble possession already in hand above a far grander one that is not yet available. Present control matters more here than the size or splendor of a future entitlement.

More broadly, the proverb favors a secure, usable benefit over a much greater benefit whose enjoyment must be deferred. Its first-person refusal makes that preference sound firm and personal.

Use it when someone is weighing whether to give up a modest advantage they already have for a much larger benefit available only later. It emphasizes the practical value of present possession without requiring the future benefit to be impossible or illusory.