break is used to exit from a for, while or do...while loop, bypassing the normal loop condition. It is also used to exit from a switch case statement.
In the following code, the control exits the for loop when the sensor value exceeds the threshold.
int threshold = 40;
for (int x = 0; x < 255; x++) {
analogWrite(PWMpin, x);
sens = analogRead(sensorPin);
if (sens > threshold) { // bail out on sensor detect
x = 0;
break;
}
delay(50);
}