Meaning
A square framed by a border, styled like a button on a screen. It simply represents a square button, an empty box or a frame.
People use it as a checkbox, a frame around text or a blocky bullet: “🔳 Option A 🔳 Option B, reply with your pick” Its partner 🔲 has the opposite colouring, and because the two names do not always match how they look, many people mix them up.