Here is a link to some basic openSUSE concepts: Concepts - openSUSE
X window (commonly known as X11 or X) provides Linux PCs a graphical environment, in which different graphical desktops can run (giving the user a graphical appearance that is typical of the OS of modern computers, such as seen in the Mac, or in MS-Windows). Without X window one would only have a simple ASCII prompt/shell type environment on their Linux PC (with no graphics)
When one starts one’s PC, if it successfully boots to KDE or to Gnome desktop, then one has booted to X.
If one first boots to a full screen text mode (run level 3) then one way to start X window is to type: startx
There are also other ways.