You set lowest number say 3.5 then most of the background will be blurred and very small area will be in focus. As you increase the number say upto 8.0 then background blur will reduce and more area will be in focus.
Above shots from VCheng is an excellent example. He was just changing the f number and see the effect on background 
In his first shot it is higher f number, more blue gems are in focus and background isn't much blurred. In the last shot, hardly one or two blue gems are in focus which must be the lowest f number possible and background is very much blurred.
Keep all settings same (use aperture priority mode for this) and take a series of shots of exactly same scene with changing f number from lowest to highest you will see the impact 
