The Arduino Uno uses the ATmega328P microcontroller, you can read the specifications in the datasheet.
If I remember it right, about 3.5V to 5.0V is HIGH.
You may not apply a voltage that is lower than 0V or higher than 5V to a pin. If you do, the current may not be higher than 1mA.
That 1mA can be used to calculate an extra protection resistor.
Suppose you have a signal that can be 0V or 5V, but a pulse of 10V might occur. The protection resistor is (10V-5V)/1mA=5k. So the protection resistor is 5k6 (between the signal and the Arduino input pin).
If you have a wire with 12V and let that accidently touch the Arduino board somewhere, that will fry your Arduino board.