Being "Lightning Ready" means an app (written for "Classic") works as designed when "Lightning Experience" is turned on.
Salesforce's definition of "Lightning Ready" as of November 2016 is:
100% of your End-User Use Cases must work as expected in Lightning Experience. (It is not necessary to re-style your app or use Lightning Components.)
For complete details visit https://goo.gl/j3sfRf.
