You would type a character on a keyboard, and the computer would generate that character on the screen, usually in glowing greenish phosphor against a dark background.
Caffeine can inhibit what are called phosphor diastasis, and if you stop these enzymes from working properly, then you can actually cause the body to burn more fat.
A phosphor is a substance that absorbs energy from another source, like a light for example, and then emits that energy as visible light, which is the glow we see.
Inside the glass vacuum tubes, electrons were fired at a screen covered in red, green, and blue phosphors, a type of material that glows when hit by something like electrons.