GPIO is general purpose input and output pins, which are used for digital input and output pins means high voltage and low voltage in any circuit board.
In raspberry pi, there are 40 (26 in some models of raspberry pi) pins. Where some are fix for use as a power and ground and remaining is use for input and output.
These pins are specially use in microcontroller for get input from any sensor or send output to any machine or motor for control robot or other device. In raspberry pi, high voltage is 3.3v and low voltage 0 for input and output GPIO both.
Some pins are used for UART in Raspberry pi.