What does "The sensor is operating under critical or fatal conditions" means?

What does "The sensor is operating under critical or fatal conditions" means?

In OpManager. My ESX shows a critical error
  1. Hardware Sensor : storage - Memory - The sensor is operating under critical or fatal conditions. This may directly affect the functioning of both the sensor and related components.
No idea what that mean. 
              New to ADManager Plus?

                New to ADSelfService Plus?