What temp would be a "simmer"?

I'm trying to cook some oxtails in my cast iron pot and the directions call for leaving it on the stove for about 2 hours on a "simmer". But I'm not exactly sure what number to put it on so I won't accidentally overcook it, nor am I sure what exactly I should be testing for to make sure it is "simmering" and not boiling and potentially getting burnt or spilling out of the pot. Usually I'd prefer just tossing it in the oven at a given temperature but i don't see any oven recipes online.