Represented by the blue icon with the smiling face, the macOS Sierra Finder is the home base for your Mac. You use it to organize and access almost everything on your Mac, including documents, images, movies, and any other files you have. However, sometimes you need to restart it if it's acting ornery or if you’re installed an app that requires a restart. To do so:
- Open the Finder.
- Click the menu and hold Shift on your keyboard.
- Click Force Quit Finder.
Or you can:
- Hold the ⌥ (alt/option) key on your keyboard.
- Right click on the Finder icon in your Dock.
- Click Relaunch.
(This how-to is based on my experiences and info on Apple's support pages — where the images come from.)