Muscle Sensor v3 problems

I am still not getting the meaning of "secure".

My problem is listening to PIN A0 give me constant value 1023

Following is my Arduino Code :
int sensorPin = A0; // select the input pin for the potentiometer
int ledPin = 13; // select the pin for the LED
int sensorValue = 0; // variable to store the value coming from the sensor

void setup() {
// declare the ledPin as an OUTPUT:
Serial.begin(9600);
pinMode(ledPin, OUTPUT);
pinMode(sensorPin,INPUT);

}

void loop() {
// read the value from the sensor:
sensorValue = analogRead(sensorPin);

Serial.println(sensorValue);
delay(100);
}