In general, F12 serves as a shortcut to open developer tools in most web browsers. Additionally, F12 can be used for other purposes in different applications, like opening the "Save As" dialog box in Microsoft Office programs.
When you press F12 in browser, it opens console, which is used by programmers to debug their code and the panel opens will have many tabs. One of which will show the HTML dom tree created by the browser engine while rendering the web pages.