Active Record Migration Rails

Migrations can manage the evolution of a schema used by several physical databases. It's a solution to the common problem of adding a field to make a new feature work in your local database, but being unsure of how to push that change to other developers and to the production server. With migrations, you can describe the transformations in self-contained classes that can be checked into version control systems and executed against another database that might be one, two, or five versions behind.

Running migration:

where MyNewMigration is the name of your migration. The generator will create an empty migration file timestamp_my_new_migration.rb in the db/migrate/ directory where timestamp is the UTC formatted date and time that the migration was generated.
There is a special syntactic shortcut to generate migrations that add fields to a table.

 rails generate migration add_fieldname_to_tablename fieldname:string


class AddFieldnameToTablename < ActiveRecord::Migration
def change
add_column :tablenames, :field, :string
end
end 

To run migrations against the currently configured database, use rake db:migrate. This will update the database by running all of the pending migrations, creating the schema_migrations table (see “About the schema_migrations table” section below) if missing. It will also invoke the db:schema:dump task, which will update your db/schema.rb file to match the structure of your database.
To roll the database back to a previous migration version, use rake db:migrate VERSION=X where X is the version to which you wish to downgrade. Alternatively, you can also use the STEP option if you wish to rollback last few migrations. rake db:migrate STEP=2 will rollback the latest two migrations.

For more detail  : Active Record Migration