By default the gadget counts matching work items. Choose a number field instead and it adds that field up: story points, an estimate, a cost, anything numeric.
If two custom fields share a name, and on most sites some do, the list shows the field id next to each so you can tell them apart. Picking the wrong one makes a total quietly too small, so the gadget refuses to guess for you.
Click it and Jira opens the work items it counted. A number you cannot open is a number you cannot act on.
Set a target and the bar fills toward it. Say which way is good: finished work should go up, open bugs should go down, and the colour follows whichever you picked rather than assuming that more is always better.
A total stops at the first 2,000 work items and the gadget says so on its face. It will not show you a total that is silently wrong.
It refreshes itself every five minutes. If a query fails it prints the reason in words instead of leaving a spinner turning on a screen nobody is standing at.