By pressing certain key combinations, you can do things that normally need a mouse, trackpad, or other input device.

To use a keyboard shortcut, press and hold one or more modifier keys and then press the last key of the shortcut. For example, to use Command-C (copy), press and hold the Command key, then the C key, then release both keys.

Using keyboard shortcuts can dramatically improve your productivity.

Press this keyTo do this
Command-XCut the selected item and copy it to the Clipboard.
Command-CPaste the contents of the Clipboard into the current document or app. This also works for files in the Finder.
Command-ZUndo the previous command. You can then press Shift-Command-Z to Redo, reversing the undo command. In some apps, you can undo and redo multiple commands.
Command-ASelect All items.
Command-FFind items in a document or open a Find window.
Command-GFind Again: Find the next occurrence of the item previously found. To find the previous occurrence, press Shift-Command-G.
Command-HHide the windows of the front app. To view the front app but hide all other apps, press Option-Command-H.
Command-MMinimize the front window to the Dock. To minimize all windows of the front app, press Option-Command-M.
Command-OOpen the selected item, or open a dialog to select a file to open.
Command-PPrint the current document.
Command-SSave the current document.
Command-TOpen a new tab.
Command-WClose the front window. To close all windows of the app, press Option-Command-W.
Option-Command-EscForce quit an app.
Command–Space barShow or hide the Spotlight search field.
Control–Command–Space barShow the Character Viewer, from which you can choose emoji and other symbols.
Control-Command-FUse the app in full screen, if supported by the app.
Space barUse Quick Look to preview the selected item.
Command-TabSwitch to the next most recently used app among your open apps. 
Shift-Command-5In macOS Mojave or later, take a screenshot or make a screen recording.
Shift-Command-NCreate a new folder in the Finder.
Command-Comma (,)Open preferences for the front app.