Yeah, so it looks like the migration has already been run.
You write the migration, you run 'rake db:migrate', the migrations are applied to the database. 'rake db:migrate' then won't do anything until you create a new migration.
A migration is like a set of instructions of what should be done to a database, so each one is only run once. If you were working in a team, the migrations can be kept with the project and then team members can easily run them against their own databases.
schema.rb then keeps track of what your database looks like (and also lists the number of the latest migration in the :version hash).