large improvement of what is wrong, corrupt, unsatisfactory
Reform means a useful change, or sometimes reversion to a pure original state.
“Reform” means changes. The UK government frequently uses the term "reform" to describe changes to public services, such as the National Health Service. But these changes are not accepted by all people as useful.
A note about grammar: when used to describe something which is physically formed again, such as re-casting it in a mold/mould, or a band that gets back together, the proper term is re-form (with a hyphen), not "reform".