A microcontroller is basically a computer on a chip. It contains a processing unit (CPU), memory (ROM and RAM), serial communication ports, analog to digital converters (ADC); they are called microcontroller because they are small and they allow to control actuators (motors and other devices).
In this example I learned to use the AnalogRead and AnalogWrite functions to measure and apply different levels of voltage to a LED respectively and how to use the serial monitor of Arduino. I used a potentiometer, that is a variable resistor, a LED's and a 220 ohms resistor. The effect of using a potentiometer to control the brightness level of a led is showed in the next video.
The Serial Peripheral Interface bus (SPI) is a synchronous serial communication interface specification used for short distance communication, primarily in embedded systems. SPI devices communicate in full duplex mode using a master-slave architecture with a single master. The master device originates the frame for reading and writing. Multiple slave devices are supported through selection with individual slave select (SS) lines.