What it means
SEO and content changes take time to show results. A measurement window keeps you from judging too early and drawing the wrong conclusion, or from never checking at all.
How to use it
Set the window before you make the change, and hold off on further changes to the same page until it closes. Otherwise you can’t tell which change made the difference.
Example
A team sets an eight-week window for a guide refresh on acme.com. When it closes, they compare impressions and rankings for the target queries against the baseline.