This is a "help feature", it shows your the PAPER, depends on you current PAGESETUP (or start PLOT dialog)
Plotter, Plotarea, Paper(size), Origin, PlotScale.. the result that area.
Just for testing: Start PLOT command, choose your Plotter, Plotarea, Extents, Center, 1:1, [APPLY] to layout & Cancel
The area changed now, right?
Paper & printable borders (the 'hidden'line inside the Paperarea) is an option, i prefer to turn it on, specially for beginners.
OPTIONS, Display, lower left corner you will find where to turn it ON/OFF
Upper left corner: [Colors] , you can set the LAYOUT color to white, a kind of white is default.
Note: Sometimes this helper tool(the paper position and size) is not up to date, but for usual it is.