What does "spin on" mean?
I found my answer in Wikipedia:


  Spin coating is a procedure used to deposit uniform thin films to flat
  substrates. Usually a small amount of coating material is applied on
  the center of the substrate, which is either spinning at low speed or
  not spinning at all. The substrate is then rotated at high speed in
  order to spread the coating material by centrifugal force. A machine
  used for spin coating is called a spin coater, or simply spinner.