What are Command Bar Actions
A Command Bar Action is an action you can take in Arc just by typing into the Command Bar (AKA the new tab button or Command-T) .
In addition to creating new tabs in the Command Bar, you can type common actions you take in Arc, like Pin Tab or Add Split View, to take actions in an instant and without moving your fingers to your mouse.
How to Use Actions
View and select actions by hitting Command-T and entering "Tab" to filter the command bar for actions.
What Actions Should I try
Any action you can take in Arc can be also done in the Command Bar. But a few great ones to try are:
Pin to [Space Name]
New [Folder, Note, Easel, Space]