What do "conditional admission" and "validated with reserve" mean?

Short answer: You are accepted if you send something later, like your final degree certificate. In most cases the reserve must be cleared before you apply for the visa.

The visa can be filed with a pre enrolment validated with reserve in only two cases (MUR circular):

You do not yet have your final certificate. Then you need an official provisional certificate; a self declaration does not count.
You must sit a language test or admission exam in person. Then the embassy can issue a special 100 day study visa.

If the university does not validate your pre enrolment, the visa is refused automatically.

This page explains official rules in plain language. I am not an immigration adviser and I do not give visa advice. For that, always use official government sources. Rules change: confirm with your university, the embassy and VFS before you act.