You’re supposed to “be IN the world, not OF the world.”

It means that while you technically live here, among other people, you’re supposed to hold yourself apart from whatever those people are doing. It leads to a whole “that’s not meant for me” mindset where you avoid learning new things because that might make you too knowledgeable and worldly. Like, you could befriend non-Mormons, but it really should be for the purposes of converting them, not to get to know them or have a diverse social circle. You shouldn’t let any of THEM influence YOU in any way. Mormons are weird.