To click twice in rapid succession on a mouse button. If you double-click on an application icon, Windows will try to run the application. If you double-click on a file, Windows will try to start the application that was used to create the file. A double click has to be done fast enough for Windows to realise what you are doing. To practise, use the utilities that come with your mouse. As a tip, rest your hand on the mouse with your finger resting gently on the left-hand button. Do two short presses in quick succession.
If you try to hold the mouse too tightly or press down too far, you won't be able to double-click fast enough. If you move the pointer over an icon and do a single click, the icon will be selected and you can change the icon's properties. Lastly, a double click normally only refers to the left-hand mouse button, since the right-hand button is used to control the properties of an object. (See also Drag and drop.)
