What does it mean?
General Purpose Input Output. These are generic pins on an integrated circuit whose behaviour, including whether it is an input or output pin, can be controlled by the user at run time.
Lets look at the other terms:
Integrated Circuit: is a set of electronic circuits on one small plate (“chip”) of semiconductor material, normally silicon.
Input / Output: this is often written as I/O (or io IO) Inputs are the signals or data received by the system and outputs are the signals or data sent from it.
Run Time: This is when the program is running. It begins when a program is opened (or executed/launched) and ends with the program is quit or closed.