Imagine a window screen, a squeegee and a bucket of airplane glue. Spread the glue on the screen and let it dry. You have made the screen impermeable. Now lay a blank T-shirt on a table, place the screen over it and dump a bucket of water or turkey gravy on the screen. Nothing will pass through to the shirt. You could even dump a cup of paint on the screen, drag the squeegee through the paint puddle and try to force some through to the shirt. But the shirt would remain clean.

Now take a pin and poke a few holes in the coat of dried glue on the screen. Make some big holes and some small ones in a pattern. Or spell your name. Then squeegee a glob of paint across the pattern of holes. Bingo - you screen-printed that pattern on the shirt that was lying beneath the screen!

That's the process. Upgrade it with screens of differing porosity for differing effects, special paints that are thick and easier to handle and engineered frames to keep the screens rigid. Use those components in complex assemblies that perfectly calibrate both screen and product so a series of colors can be applied with perfect registration. It's still screenprinting. Each color is applied separately by squeezing it through a screen. During the pre-production process each screen is custom made to apply in exactly the right place and with the right density a specific color used in the design.