Meaning
A heavy, slanted cross, the multiplication sign drawn as a bold X. Its literal meaning is times, as in multiplying one number by another.
Beyond maths homework, it is widely used to announce collaborations or crossovers, as in “Coffee ✖️ comics night this Friday”, and to show combos or quantities like “✖️2”. Some also use it as a plain “no” or “close”, but ❌ is clearer for that, since ✖️ looks neutral and calm.