As time goes on, it's clear texting isn't a way to communicate, but rather a modern way to ignore people more efficiently—especially with "mooning." You probably know about "ghosting," suddenly cutting off all contact to avoid a breakup conversation (or any conversation ever again). But what about people you just want to talk to less? "Mooning" is the rude but totally logical practice of silencing someone because they're annoying and/or unimportant. It could have already happened to you, and you'd have no idea.
Put simply, a mooner is muting you. (Older iPhones had a little moon symbol that came on when you muted someone, hence the term.) You can still text them, but it won't show them a notification, or make a noise, or light up. Basically, you've been relegated to a human spam folder, but unlike when you're blocked, it won't tell you.