The Apple Dock is a convenient feature of MacOS that allows users to access frequently used applications and documents with just one click. To pin an application or document to the dock, simply right-click on the icon you wish to add and select 'Keep in Dock'. The item will remain in the dock until it is removed manually by dragging it out of the dock while holding down the Command key. Additionally, items can be rearranged within the Dock by clicking and dragging them into different locations. This makes finding your commonly used programs much more efficient.