OL" stands for "overload" on a Fluke multimeter. It appears on the display of a multimeter when the input signal exceeds the measurement range of the multimeter. This can occur when the multimeter is set to measure a voltage or current that is higher than its maximum measurement range, or when the multimeter is measuring a sudden and large spike of current or voltage. When the "OL" message is displayed, it is an indication that the multimeter is not able to accurately measure the input signal. This can happen in many situations, such as when the input is too high, or the multimeter is not properly connected to the circuit, or the fuse is blown, etc. To avoid "OL" message, you should select the correct range for the measurement, and make sure the multimeter is properly connected to the circuit before taking a measurement, also check that the fuse is not blown, and use proper protection if the measurement is expected to be high. It's important to note that if you see the "OL" message frequently, it could indicate that the multimeter is damaged and should be sent in for repair or replacement.