Someone playing one of my games has asked my permission to be allowed to stream some of my games on Twitch. I fear I am in unknown territory here. Can someone advise me as to the significance of this, and whether giving permission is a good/bad idea?

You mean someone has offered to give you free publicity, in front of a potentially huge audience? Congratulations!

If you're worried about the legal significance, I'm of course no lawyer, but I think you're being asked for something called "performance rights". That's totally a thing, and Nintendo for instance has been known to crack down on such uses, including Let's Play videos of their games. But unless the streamer will be making $$$ and not paying you a cent, it means very little. So relax and watch the show, whenever it happens.

Thank you, felixp7, I'm reassured!