What op means is this...
Consider your car, it is full of electronics, most of which you control, you turn lights on and off with switches, turn the radio on and off, volume up and down, change the station, change the climate control temperature, the windscreen wipers, all of it is making a circuit and requires you to flick a switch or press a button to do it.
These make sense though, you turn the switch for your headlights and the circuit is complete, every time the battery is connected to the now bridged switch and onto the bulb which ks now powered, so on.
Your PC though, if i press the R key now, it types the letter R, if i press it in CoD, it reloads, if i press start and R, it opens run. The same mechanical action brings out a new action on screen, but how?
In short, do a bit of research into transistor gates and storage media. You pressing a key does bridge the gap to create the same circuit every time, however the outcome differs depending on different criteria.
Are any other keys pressed?
What software is currently has the attention of the keyboard?
How are the keys mapped for that software?
Are any other programs open that, with the right key combination, can take an input from the keyboard?
Depending on memory state and how the transistors interact with the flow of current changes the outcome.
The "switches" are the transistor gates which can swap or amplify current, depending on their current state and what is put in.