An address bar usually refers to the text box in your browser that shows the URL location a browser tab or window is currently on or viewing. A user can manually fill in the URL he wants to visit from here. Or he/she can visit a site in another way (e.g. by clicking a link) and the address bar will show the resulting URL.
Sometimes, the address bar refers to a path that is shown in a file explorer utility.
Note that under some circumstances, the address bar also functions as a search bar if the text that is entered is not recognized as a valid URL.