Candidate should obtain at least 50% marks (45% in case of candidate belonging to reserved category) in the qualifying examination what is meaning of this?

You haven't mentioned the entrance exam you are Inquiring about. But to give you an example, if you are studying M.Sc, and the minimum educational qualification to study M.Sc is B.Sc, your qualifying examination becomes B.Sc. And to be considered eligible for M.Sc entrance examination or Merit based admission, you should secure a minimum of 50% in your B.Sc. It is 50% in your B.Sc if you belong to General Category/EWS and 45% if you belong to OBC,SC,ST. Your qualifying exam is the educational qualification that is required to study the desired programme.