Client-side routing allows you to navigate around a web app without having to reload the page, using the HTML5 history API (or hash fragments at the end of a URL, in older browsers).
For a good introduction to routing, see this Stack Overflow answer.
Ractive doesn't have any built-in routing support, because it's a library not a framework (i.e. it works for you, you don't work for it).
If your app needs routing, there are a number of decent routing-only libraries available - see microjs.com for starters. page.js is a popular choice. Alternatively, if you're already using Backbone (see Using Ractive with Backbone, you could use Backbone.Router.