Meaning
A thick red ring, empty in the middle. In Japan a circle like this, called maru, marks a correct answer or a yes, the opposite of ❌, while elsewhere it is simply a bold letter O or a circle.
People use it for right answers, approvals and quizzes, for noughts-and-crosses jokes and to spell words with a bold O: “Did you bring the documents? ⭕ all set” Note that it does not mean zero; for numbers use 0️⃣ instead.