DbSchema holds its own image of the schema, independent from the database.
This image is saved to project file.
Schema Synchronization means to compare and apply the schema changes in the database or in the local project.
This allows to create the same schema over multiple databases or to upgrade the production databases to the development schema.
During schema synchronization you can view the differences and choose for each of them an action: change in the project or in database.
For each difference the SQL script can be generated.