Simply: Graphical User Interface. These are the windows and little boxes that enable control of the software you use. Behind these visual tools there are files and databases that actually
control the software. The visual tools often offer easier and quicker access to the things you want to do, but using command line, modifying config files, and other non-graphical methods often offer more control; this is the trade-off.
New users should try to stick to the graphical methods until they are comfortable with the software.