The sketch

(the code)

setup function

The setup function is the part of our code
which will be executed first

In this part we'll generally put initialization operations

e.g. serial opening, setup GPIO function, init. sensors

loop function

The loop function is the part of our code
which will be executed after setup, then run repeatedly

This is the core of our application,
here we put the operations that
the MCU will have to perform

e.g. read a sensor's value, send a signal

A microcontroller does this,
it continues to repeat an operation,
forever and ever