Screen printing is when you push ink through a gap in a screen so that it creates a pattern.
To make a screen, a stencil (positive) is made in clear plastic either by a printer or painted by hand. Next, the emulsion is put onto the screen in a dark room and then the stencil is put over the screen. Then the screen is then exposed to UV light and then the screen is washed off and the unexposed emulsion washes out.
Ink is put onto the screen and spread into the gaps in the emulsion with a squeegee. Then, the screen is put onto the canvas/medium for printing and the ink is squeezed out with the squeegee.