Safe mode is another recovery function. It's used to recover softbricks by allowing the user to boot straight into the system settings and update the system to repair it.
IIRC Luma3DS actually blocks recovery mode, it used to at least.
Regardless I wouldn't suggest messing with random stuff if you don't know what you are doing. Despite hacking the 3DS being extremely easy and recovery being equally as easy, I always suggest not going too crazy if you don't know what are doing.