Digital inputs EFI Fundamentals

I am not able to understand the concept of how the resistor "pulls up" to 5 volts. How does the resistor pull up the voltage in the digital input pin to 5 volts? I cant seem to understand the logic of switching between ground and 5 volts when on and off and why the pull up resistor wont be required when shuttling between 5V & 12V?

Maybe this will help you:

https://en.wikipedia.org/wiki/Pull-up_resistor

to learn more about electrical engineering in general you might want to start here:

https://www.khanacademy.org/science/electrical-engineering