Selecting this option ensures that all data may be examined at once. For example, in single select mode on a year filter, you can only look at revenue results for a specific year. It may be interesting to look at all reporting years at once.
This option is on my default.
In the dashboard's Edit mode, hover over a filter until its controls appear on the top right.
Click the icon (gear).
The Settings modal window appears.
In the Settings modal window click Display Settings tab.
Click In single select mode, include a '(All)' option.
Click Apply.
The All option appears in the filter box.