Meaning
A large white square, usually with a thin outline, filling the whole emoji space. It means white, blank or empty, like an unfilled tile.
It is the empty slot in progress bars and habit trackers and the miss tile in word-guessing grids on light themes: “Reading goal: 🟩🟩⬜⬜⬜” It also forms snow, paper or blank space in pixel art. On white backgrounds it can look faint. Smaller sizes are ◻️, ◽ and ▫️.