What does semi fu even mean in iphones?

I have an iphone that is semi fu and i've been told that i shouldn't update it because something bad may happen to the sim, and thanks to that i can just do limited things to it since i can't update it to the latest ios version. I don't know what to do huhu

Sounds like a hardware SIM reset tool to me. These are more common in the Chinese market where the iPhones were locked to a Chinese carrier and that the chip would deactivate the SIM reader, rendering the iOS device useless on other carriers. Why the Chinese market would use this method instead of just blocking the IMEI on the iOS device, I wouldn't know.