There's no known timeline for healing after a breakup, and much as people would say it is not okay to jump from this to that relationship, we can all agree that people heal differently and how fast depends on what the person does, wants, and for how long. Heartbreaks are emotionally challenging and stressful and require some good mental work.
Rebounds, or rebound relationships, are those that one has immediately after a heartbreak. They might have not gotten over the other person completely, but have their reasons for accepting to get into another relationship without wasting time.