Canon EOS Rebel T5i 18-55mm IS STM Lens Kit Manual (en)

To blur the background or to make everything near and far look sharp, 
set the Mode Dial to <f> (Aperture-priority AE) to adjust the depth of 
field (range of acceptable focus).
* <f> stands for Aperture value, which is the size of the diaphragm hole inside the lens.
Set the Mode Dial to <f>.
Set the desired aperture.
The higher the f/number, the wider 
the depth of field where sharper focus 
is obtained in both the foreground 
and background.
Turning the <6> dial to the right will 
set a higher f/number (smaller 
aperture opening), and turning it to 
the left will set a lower f/number 
(larger aperture opening).
Take the picture.
Focus and press the shutter button 
completely. The picture will be taken 
with the selected aperture.
: Changing the Depth of Field
Sharp foreground and background
(With a high aperture f/number: f/32)
Blurred background
(With a low aperture f/number: f/5.6)
Aperture Display
The higher the f/number, the smaller the aperture opening will be. The 
apertures displayed will differ depending on the lens. If no lens is attached to 
the camera, “00” will be displayed for the aperture.