After a long period of use you may run into an issue where your Mac freezes, or where one app freezes on your Mac.
If you cannot choose to quit the app normally due to this you will need to Force Quit the app.
If you can click on the icon in the dock, hold Option and click, which will bring up a Force Quit action. Click on that to force the app to close.
Sometimes that will not work. You may need to press Command + Option + Escape at the same time to bring up a Force Quit Menu. From here you can see apps that are not responding and choose to Force Quit.
If that fails you can press Command + Control + Eject at the same time to force a reboot. Holding the power button will do the same thing.