In GraphQL, a type is a way of defining the shape of your data. You can define a type in your Ruby on Rails application by creating a new file in the 'app/graphql/types' directory. This file should define a class that inherits from 'Types::BaseObject' and defines fields for each piece of data you want to include in the type. Each field should have a type, which can be a built-in GraphQL type like 'String' or 'Int', or a custom type that you've defined.