GPIO stands for General Purpose Input Output.The Raspberry Pi has two rows of GPIO pins, which are connections between the Raspberry Pi, and the real world.
Output pins are like switches that the Raspberry Pi can turn on or off (like turning on/off a LED light). But it can also send a signal to another device.
Input pins are like switches that you can turn on or off from the outside world (like a on/off light switch). But it can also be a data from a sensor, or a signal from another device.