Basically a transistor is a electronic switch which can have 2 states.
Conductive and insulating.
Using transistors, resistors and diodes you can build logic gates ( AND, OR, NOR, NOT, FLIP FLOPS, etc.)
Using those logic gates you can build microprocessors and other integrated circuits.
Using electric switches (in your keyboard), those microprocessors and integrated circuits (motherboard, cpu, video card, ...) and software (bios, firmware and operating system) you can type words on your screen.