Slugs are used throughout NationBuilder. They are the url address of a page. For example:

http://abeforprez.nationbuilder.com/blog 

The word blog is the slug for the blog page on the Abe for Prez website. The slug is always going to be after the backslash at the end of your website's url (the address of your homepage).

In many areas of the NationBuilder control panel, slugs are used to designate or indicate pages that you might want to direct people to, or highlight in some other way. 

Slugs will always default to be the name that you designate for a page, and spaces will always be marked by underscores (_). You can edit them to make them shorter and simpler. You want to do this so that they are easy to remember.