What makes up betterments and improvements in a condominium?

This terminology may be being used relative to a master insurance policy and coverage.

One definition might be that betterments and improvements could be upgrades to existing fixtures, equipment, appliances and so forth.

This is to distinguish these investments from 'additions and alterations', which could be adding a patio or converting a closet to a bathroom, for example.

If your question involves insurance, best practices dictate consulting with your broker or carrier to fully understand their definition of these terms.

Regardless of who's using the terms, they should define them for you so that you can fully understand them in the context of your situation.