Welcome back! I have a game where I’m sitting at a red 0.5. I understand this to mean that according to Jane’s, their military is twice as strong as our military.
I think it might be tied to the number of soldiers in Demographics. I saw an article on how that is calculated. It might be tied to “power” in the power graph. I don’t know how that is calculated.
Anyway, welcome back!
I did not have 100% certainty on this, but one game I closely tracked mine and a couple AIs power in demographic screen, it was exactly the (rounded) ratio of soldier numbers ( I do not remember what exact rounding was used, in civ it is usually rounding down, but here a mod (Bug mode?) does it)
So as with demographics screen, disclaimer as above apply: tech, pops, buildings and units all contribute. Ideally, if it is a high-stake game it is best to have a better idea what is the reason your target AI is getting the score it is getting (details of demographics explain here
https://forums.civfanatics.com/threads/the-inner-workings-of-the-demo-screen-explained.163098/)
If you want a simple rule of thumb:
green (>1) GOOD, the higher the better

red (<1) bad, the lower the worse
But remembering it is an AI, human player would typically still prevail even with a red score - maybe up to 0.5? Again, it depends. Of course, it is much safer to attack if you dominate (high green score) which I normally do
Early in game, this score can be rather misleading, since some techs like BW - or even not clearly military techs like The Wheel and Math -
contribute a lot. For example, Wheel+Math+BW is equivalent of 6 (!) archers

or 4 War chariots and a warrior.