Could someone please explain this to me?

length of rectangle is 14

and width of rectangle is 10

You'll have to compass tip on A and draw a small ark with pencil approximately in the middle above AB line, now compass tip to point B and cross the ark you made previously.
Do the same on the opposite side without making any change to the compass 
Join the lines where crosses of arks on the both side meet and then ,it's done.