Both Application Migration and Discovery Phase are different. Application Migration phase is post migration and Discovery Phase is prior to Migration.
Application Migration Phase deals with the working of your applications or business after migrating it to cloud. You visualize and make reports on the changes that are made to your application after Migration.
Discovery Phase is more about figuring out what applications you need to Migrate. Creating a report on those applications to be migrated.
Find out more about it here in the article for Phases of AWS Migration