To view available Quick Actions, you need to enable the Preview panel in Finder. To do this, open a Finder window and select the menu bar option View -> Show Preview, or press the keys Shift-Command-P.
Quick Actions for Images, Video, and Audio
Quick Actions are located in the bottom right of the Finder window, just under the preview of the selected file. These actions will change depending on the file: For images, clicking Rotate Left turns the image counterclockwise, while clicking Markup invokes an enhanced Quick Look window containing a set of markup tools.
If two or more images are selected in Finder, the Markup button will change to Create PDF, allowing you to turn the images into a single portable document. If a QuickTime compatible video or audio file is selected, Markup will be replaced by Trim. Clicking this brings up a Quick Look window with an editing ribbon to trim the file.
How to Customize Quick Actions
You've probably noticed the More... button beside the default Quick Actions. Click this and then select Customize..., and you'll be taken to the Extensions pane in System Preferences, where you'll be able to select other actions to add to Finder's Preview pane.
The actions available to you will depend on which applications you have installed and any pre-existing Apple scripts on your Mac. Apple is encouraging third-party developers to add support for more Quick Actions in their apps, but you can also create your own custom ones using the Automator app. For a useful example, check out our tutorial on how to quickly resize images using your very own Automator service.