GPIO stands for general purpose input – output pin. You can supply +3.3V electricity from the GPIO Pins and detect +3.3V electrical input from the outside. The electricity you can apply to the pins of the development boards is mostly +3.3V. The current you will get is between 10 and 20mA and the sum of the current you will get from all pins (for example) should not exceed 50mA. This means that if you want to turn things on and off directly with pins, you should use components such as transistors and relays. You can find the current that your development board gives per pin from its technical documentation.
To summarize briefly, they are GPIO input and output pins.