Good news first: This smartphone prank will not brick your iPhone forever. Bad news: It's the kind of trick a lot of people love to do to others, so if your friends learn about this, things could get annoying really fast.
Basically if your iPhone is on iOS version 10.0 to 10.1.1 and someone texts you a white flag, zero, rainbow emojis and an invisible character called VS16, your phone will go dead. Nothing will work, including the home button. This apparently happens because the phone's older software becomes confused and can't decide what to render, so it says screw it and stops. That's right, these emojis essentially make your phone go on strike for a few moments.
Fortunately it doesn't do any known lasting damage to your device. Even better, if you just update your software to the most recent version available—10.2, with 10.2.1 coming soon—it won't work. There is a similar prank that could cause issues even with the most recent iOS, but it isn't as annoying or likely to induce a few moments of total panic.
The thing about this prank is the person who pulls it, as you'll see in the video, needs to be pretty committed to the bit. To properly execute the prank, one has to sign into iCloud, put the characters in Notes (online-only version) then send from a phone linked to the same account.
There are actually a number of iPhone pranks out there that could be used to mess with your friends, but we recommend avoiding them in the interest of not getting your ass kicked.