To overreact is to answer something with too much feeling, or by doing something the problem did not need. A small delay, a short message, a tiny mistake — and someone shouts, panics, or makes a huge new rule. Parents, bosses, and friends all get accused of this. It is everyday English, a little critical, not very formal. The “too much” is in the size of the feeling or the size of the action. The first problem can still be real. Though actually, what counts as too much depends on who is speaking. The person who overreacts often feels they are only being careful; the other person feels the storm is too big for the rain. You will also hear it about groups and governments, not only about one angry moment.