Whenever you add or change a table in the Schema Designer, the changes will only be applied to the in-memory schema. The actual postgres database will never be touched by the schema designer directly.
You can think of this like in a git workflow: In git changes are only applied to the repository history when you do a git commit. In Thin Backend this 'git commit' is running a database migration.
In Thin Backend a database migration is a list of SQL statements that will be run inside the postgres database when the migration is run.