The lesson
You can add a filter two ways.
From a chart: click a bar or slice to select it, open the menu in the small box that pops up, and click Add to Filter. Clicking alone only selects.
From the Filters button at the top of the dashboard: click + Add Filter, search for what you want, and click Apply. Use this when what you want isn't on a chart.
Your filters change only your view, and they apply across every tab. A blue dot on the Filters button means a filter is on, so check for it if your numbers look low. Reset All clears everything.
Some tiles have their own tile filter, set by whoever built the dashboard, and it always overrides yours. If one tile won't change, open its menu and click Underlying Filter.