Terminate a function and return a value from a function to the calling function, if desired. return; return value; // both forms are valid value any variable or constant type A function to compare a sensor input to a threshold
int checkSensor() {
    if (analogRead(0) > 400) {
        return 1;
    else {
        return 0;

The return keyword is handy to test a section of code without having to "comment out" large sections of possibly buggy code.
void loop() {
    // brilliant code idea to test here

    // the rest of a dysfunctional sketch here
    // this code will never be executed