A densifier is a chemical which hardens a concrete floor, strengthening it against natural elements, foot traffic, moisture, acids, scratches etc.  In many circumstances a densifier is a better solution to a protective coating, because coatings wear off and must eventually be re-applied.  By hardening the concrete itself, you never have a coating to re-apply. 

When maintained, a densified concrete floor can maintain it's smooth finish for 10 years or more.