Can anyone explain simply what is a dock applet? I installed Ubuntu Budgie on my laptop and realized that I have this app installed. It says that it is dock applet. But unfortunately I don't know what is a dock. I know that an applet is a small application but what "dock" means? What can I do with this applet? Purpose of this?
A dock is a row of icons that you can click to launch an application if it is not yet running, or switch to that app if it is already running. It was, I guess, Apple that first came up with the dock at the bottom of the screen. This has become a popular desktop accessory, since, replacing the more traditional window buttons to switch application, and separate launchers to start applications.
plank indeed is a dock application. It does not belong directly to the Budgie desktop: it is an independent application. However, the developpers of Ubuntu Budgie include it by default in their build of the Budgie desktop. It is placed at the left edge of the screen. Thus, if you did not yet modify your desktop, the application icons on the left of the screen are the plank dock.