People have already correctly answered that in this community there are no stupid questions, you’re supposed to be able to ask anything without ridicule. But if you want to permit the general existence of stupid questions, here’s the definition I’ve always used.
A stupid question is one you can easily answer yourself.
As in, any question due to real lack of knowledge is not stupid, but if you can answer your own question just by thinking about it for a second, then it wasn’t necessary to ask and therefore "stupid”.