Meaning
A hand with the fingers split into two pairs between the middle and ring fingers, the salute of the Vulcans from Star Trek.
Fans use it with the phrase “live long and prosper”, and more broadly for nerdy greetings, sci-fi chat and friendly goodbyes. Not everyone can make the gesture, which is part of the fun. Skin-tone versions exist. Pair with 🚀 or 👽. Example: “See you at the convention, live long and prosper 🖖”.