When a controller is trying to communicate with the real world, the most simple way of talking is in the form of “True” or “False” i.e “High” or “Low” in the controller language.
The controller gives the output in the form of high level or low-level voltage and also senses any input in the same way. When the voltage level is above a certain value, then the controller understands it as a “High” or “True” and when the voltage level is below a certain value then the controller terms it as a “LOW”. So, when the controller gives out the voltage on the pin, it is termed as Digital Output and when it senses the voltage on the pin, it is termed as Digital Input. The Arduino has 14 digital Input/Output pins. (D0-D13).