Screen resists determine where we don't want the ink to go through the screen, what is not to print. This is the basis of the silk screen process.
It used to be that a silk screen artist had to cut stencils, usually from a green film that they adhered to the screen with a chemical. Some screen printers still do this, but there are more options now - often easier to apply, and allow for better detail than a stencil cut.
The most basic resist is Speedball Screen Filler, which can be brushed directly onto a screen to block areas that the artist does not want to print, the negative. Or if he prefers, a Drawing Fluid can be painted on the positive, the screen is coated entirely with Screen Filler, and the Drawing Fluid is washed away to expose the screen beneath, the positive.
Speedball Photo Emulsion is activated with Sensitizer to become sensitive to light. Light exposes the resist around the 'shadow' of the image to be printed. Photo Emulsion allows for the tightest detail of all the resists.