Depending on how smart it is, it could be indicating:

No ground, as above.

Ground and neutral swapped.

Live and neutral swapped.

No neutral (very unlikely, I suspect, but not impossible.)

You can get little plug in widgets which will show on lights which of these is the case, as well as other issues. Something like this for example.

I'd suggest it's something useful to have in your toolkit as a general test device. The most likely probability is lack of a ground, but any of the others are possible and can cause a hazard if something goes wrong.

pca