While developing you sometimes need to change the database. But to be able to change the database on the server while deploying instead of doing these changes manually we will use migrations.
Migrations in the framework are handled by the DoctrineMigrationsBundle.
Basically it is simple. The only thing you have to do when you need to change the database structure is to change the annotation/configuration/... and run:
app/console doctrine:migrations:diff
This will generate a migration-class which you can commit along your changes. Of course you will need to run the migration to update your own database:
app/console doctrine:migration:migrate