It allows you to write your domain classes in a Groovy DSL, called GORM http://grails.github.io/grails-doc/latest/guide/GORM.html and generate SQL and SQL migrations as you change your classes.
Here's the Getting Started page from the documentation http://grails-plugins.github.io/grails-database-migration/do...
There is a standalone GORM that allows you to use the Groovy DSL without Grails, I'm not sure how the Migration plugin could be made to work with that.