Meaning
The # sign on a square key, like a button from a phone keypad. The symbol is known as the hash, number or pound sign, and on keypads it often confirms an entry.
Its biggest job in chat is announcing hashtags and topics, or numbering things in a playful way: “#️⃣ Throwback to our first class trip” It also suits phone menu instructions and codes. Technically it is the # character combined with a keycap frame, which is why it looks like a button.