By default, Rails applications build URLs based on the primary key -- the id column from the database. Imagine we have a Person model and associated controller. We have a person record for Bob Martin that has id number 6. The URL for his show page would be:
/people/6
But, for aesthetic or SEO purposes, we want Bob's name in the URL. The last segment, the 6 here, is called the "slug". Let's look at a few ways to implement better slugs.