The text editor area will have the following lines of code:
void setup() {
}
void loop() {
}
The setup() method is the first thing that is called when it is powered up. Everything in this area will run exactly once. This is the best place to initialize all your input and output pins; if you're following C++ syntax, you could also use define statements or global variables as well.
The loop() method loops the code inside it repeatedly unless there's a stop command in it or you remove power from the Particle board. This is where most of the code is located.