What is Aperture ?

It's the size of the hole through which light travels into your camera.

Left to right, small apertures (f/5.6) to big apertures (f/1.8) shown above.

Rated by f-stop:

• f/1.8 or f1.8 = wide opening / big hole / big aperture.
• f/5.6 or f5.6 = small opening / small hole / small aperture.

Related: Depth of Field (DOF), Shutter Speed, ISO.

• low f-stop (f/1.8) = wide aperture, more light, high shutter speed, small DOF, allows low ISO.
• high f-stop (f/5.6) = small aperture, less light, slow shutter speed, large DOF, may require high ISO.
  
  

  Depth of Field	digital-photography-school.com
  Small Aperture, large DOF, everything sharp.	Large Aperture, small DOF, background blur.

In short:

• To blur backgrounds and have a small DOF, lower the f-stop to have a wide Aperture.

 

References:

• http://www.mir.com.my/rb/photography/fototech/apershutter/aperture.htm
• http://www.cambridgeincolour.com/tutorials/camera-lenses.htm
• http://en.wikipedia.org/wiki/Aperture