Short answer: “hard science” usually refers to fields seen as highly mathematical, experimental, and precise, such as physics or chemistry. “Soft science” usually refers to fields studying complex human or social systems, such as psychology, sociology, or anthropology. The distinction is common, but often misleading.
The problem with the hard/soft language is that it can make some fields sound serious and others vague. In reality, different subjects face different kinds of difficulty. Physics can often isolate variables with extraordinary precision. Social science often studies people, institutions, cultures, and behaviours that change because they are being studied.
What the labels miss
A field is not weak simply because its subject is complex. Human behaviour is difficult to measure because people are not particles. They learn, lie, forget, interpret, adapt, and respond to context. That does not make research impossible. It means methods have to match the subject.
At the same time, fields sometimes called soft sciences can suffer from real weaknesses: small samples, measurement problems, overgeneralisation, and replication issues. Those criticisms should be made specifically, not through a lazy label.
How to judge a field
Ask whether the field uses methods appropriate to its questions. Are claims testable? Are measures valid? Are uncertainties acknowledged? Are findings replicated? Are alternative explanations considered? Those questions are better than asking whether a field feels hard or soft.
The best science is not defined by sounding like physics. It is defined by disciplined inquiry, evidence, transparency, and correction. Some questions are harder to answer precisely because they are closer to human life.