📄️ Input vs output
A system is only limited if it performs operations on constant things, meaning that it's not worth the hassle to use a microcontroller to print the number 5 forever, to add the numbers 2 and 6 for the rest of its lifetime, to turn a buzzer on indefinitely.
📄️ Digital vs analogue
The nature of all signals is categorized into two categories: Digital and Analogue. These signals can be light waves that are emitted from a light source, which are analogue signals, or signals a push button sends the microcontroller when it's pressed, which is an example of digital signals. The graph shown below is for an analog signal and its digital version. Computer systems, like your PC or microcontrollers, deal internally with digital signals ONLY, so analog to digital converters can prepare the signals for these devices.