What does it mean to read from the PIN register?

Question

Programming

Writing to the PORT register is understandable. We assign a low or high potential to the output, having previously configured DDR = 1. Is it possible to write to the PIN register? Or is it already recorded before us and it remains only to read? In general, the word read from the register is a play on words? I don’t understand what they mean when they say read from the register? \
Here a high potential is applied to the input or the input is pressed to zero. What is the name of this action?
As I understand it, by applying to the input or pressing the input, the register reads its state. That is, the action of applying to the input is the signal level, and the reading is the result of the measurement. And where do they read from the register?

Answer 1

As far as I understand, port is no different from pin. There is a special microcircuits (electronics engineers often call them drivers) that are connected to an external bus (it doesn’t matter which one - discrete, analog, digital). This microcircuit communicates with the processor already through the internal line.
But the special processors have the ability to connect the input directly.