A scientific hypothesis is a proposed explanation of a phenomenon that can be tested by experiment or observation.
Formulating hypotheses is part of the scientific method. The scientific method in a simplified form:
Observation of a phenomenon → proposing an explanation (hypothesis) → using the hypothesis to formulate predictions → performing tests or further observations of phenomenon → rejecting or accepting hypothesis as explanation for the phenomenon.
When a hypothesis have been tested many times and by independent scientists, and there is no inconsistency between observations and predictions, the hypothesis becomes a theory.
The strength of the scientific method is that theories are not dependent on the individual scientists, as in principle everyone can redo the test by themselves to confirm (or reject) the theory. In fact a theory or hypothesis is only considered scientific, if it is possible to prove it untrue. For example, when a theory make predictions about the result of experiments, results different from those predicted would contradict the hypothesis and prove it untrue.
Proposing explanations or hypotheses that cannot be falsified, is not scientific.
Even though theories are the accepted scientific consensus, they are not laws. Therefore if a theory fails to explain new experiments or observations it is abandoned, and instead new hypotheses are formulated and tested.