I don't know the term either. I don't think it is established baking slang, so it is bound to vary between recipes, should you find it in another one at all. But if you got it from a bread recipe, it must be because you need optimal conditions for your yeast. The optimal temperature for yeast rising is 35°C, with rising being too slow below that (but it will still happen, even at 4°C in the fridge!) and not possible at 40°C and above, where leavening action gets too low for practical purposes (and at some point, the yeast dies).
This is a nice representation of the amount of CO2 produced by yeast (which correlates well with leavening) at different temperatures. The difference between the low effective temperature (25°C), the optimal temperature (35°C) and the upper limit of the effective temperature (40°C) is not big, so I don't rely on my imperfect senses and always use a thermometer when making yeast dough.
But you are writing that you want a "rule of thumb", so you probably don't have (or don't want to bother with) a thermometer in your kitchen. In this case, you can still have your bread rise well. The literal thumb is a bad choice, as it is quite insensitive, but the trick our grandmas used to gauge the temperature of infant food is still valid: use your elbow.
The skin of the elbow is very thin, and it will very well notice the difference between a 35°C liquid (which feels roughly the same temperature as the elbow, remember that 37°C are normal inside the body, not on the skin outside) and a 40°C liquid (which feels too warm). If you were to use your fingers or the back of your hand, you wouldn't be able to tell the difference well enough, and will risk overshooting and killing the yeast. There are possibly other body parts which can feel the difference (I'd guess the tongue, if you don't scald it daily with hot drinks), but hygienewise, the elbow is probably better.