I'm using a tm4c123g tiva launchpad, but I believe these questions are general.

1. If a pin is set to output, what does it mean to "apply a different voltage on it"? This is followed by reading the post: [What happens if a voltage is applied to an output MCU pin?](https://electronics.stackexchange.com/questions/182551/what-happens-if-a-voltage-is-applied-to-an-output-mcu-pin)

Does that mean using a second power source on the same breadboard?

2. If I output a pin as GPIO high to drive an LED, why do I still need to connect the 3.3v to breadboard + rail? Shouldn't the pin itself provides enough current? This is followed by reading this schematic:

[https://users.ece.utexas.edu/~valvano/Volume1/E-Book/C8\_SwitchLEDv2\_files/Fig8\_7.jpg](https://users.ece.utexas.edu/%7Evalvano/Volume1/E-Book/C8_SwitchLEDv2_files/Fig8_7.jpg)

3. The datasheet doesn't go over the circuits of pins in detail. What exactly are pins? I'm asking this question in the context that they are pretty much black boxes to me and I don't know which should be avoided to damage them in a circuit.