The Dock is a graphical user interface element on macOS used to manage applications in the macOS environment. The Dock appears as stage for icons of pinned apps the user has placed there; it also covers Stacks, used for displaying folders, to the right of the line. The Dock can also initiate Mission Control. The color of the Dock can be light or dark.
The Dock serves as an application manager, allowing you to open, close and switch applications. A running application is indicated by a dot underneath it, and clicking on it will bring up the last used window, while holding down on it will bring up all the windows of that application layed out in a grid, a feature called Mission Control. Stacks also come under the Dock. To the right of the line are more icons, known as Stacks (as well as the Trash icon). When clicked on, these open up fans of up to 10 items, or grids which display all the items in a folder. You can also browse through the folders inside one another in the Stack.
When Simple Finder is activated, the Dock only lets you switch to applications that are open, and three folders that cannot be moved.