Thats your printable area..

You choose a papersize..

the papersize-definition is length x width and non-printable border left/right/up/down

Think about: Not every printer can print up to the outer paper border!

If you want to edit the printable border you have to re-define your papersize definition,

or if you using a standard papersize format, create your own custom papersize (with another name).

Or simply: Choose the well fit papersize, big enough for your wanted viewport, frame and titleblock.