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5.22 - Overload
The SDR-IQ is capable of operating properly with signals of up to two volts at the input to the A/D. Above that level, it becomes "blind" to what is happening at the antenna.
allows you to (additionally) monitor for FFT math overflow, which is not the same as normal overflow — it doesn't always mean that your SDR is being overloaded with a strong signal. The same overload indicator is used, and signal overload will still cause it to illuminate.
The indicator uses different colors to tell you what is going on. Unlit, there is no overload. 
OVL the overload is coming from the SDR. 
OVL, the overload is coming from the FFT math; 
OVL, from both.
So OVL shows you when electrical overload is becoming a problem by turning or flashing red. If, on the other hand, you see OVL, this means the signal is very large, perhaps even large enough for the mathematics used to handle it to reach its limits, at which point distortion and other audible problems will ensue — but the signal is not overloading the RFSPACE SDR. If both electrical and math overload are detected, you'll see OVL, and at that point you will almost certainly see and hear other effects — it's not a good thing.
A few flashes aren't of much concern, but if the indicator turns red and stays red, you are receiving signals near, or past, the two volt level where the SDR-IQ can operate properly.
In case of electrical overload with an RFSPACE SDR, type s, press , or use the menu to open the SDR dialog, and engage additional RF attenuation until OVL or OVL goes out. In the special case of math overload with the SDR-IQ, you may also be able to reduce the IF gain until OVL goes out.
If you see OVL with any other type of SDR, likely you need to do something with your antenna system to reduce the signal. I can't really advise you beyond that point other than to say constant overload isn't good for radio electronics, SDR or otherwise.
If you have an antenna such as the RF-PRO-1B active magnetic loop antenna which is directional, and your overload problem is a consequence of a local AM or other station, you can aim the antenna's NULL, or weakest receiving orientation, at the problem station. This works extremely well, particularly during the day. One caveat for the RF-PRO-1B active magnetic loop antenna is that you need to mount it away from metal objects, or the sharpness of the NULL will suffer. I speak from personal experience here.
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