The reporting period dropdown menu in the top-right corner of most Insights pages indicates the time period of the statistics on your current page.
You can pick between Today, Yesterday, Last Week, Last 2 Weeks, Last 4 Weeks, All Time, and Custom Range. With each time period, the statistics in your dashboard will change.
On your Insights Dashboard, your reporting period is defaulted to Last 4 Weeks.
If you'd like further details on specific reporting periods, create a custom dashboard!