Route globbing is a way to specify parameter from request should be matched to all remaining parts of a route. For example in Rails 4:

get 'users/*xyz' => 'users#list'

When client send GET request with URL /users/foo/bar, Rails will automatically match to action list in controller users with params[:xyz] equals foo/bar. The fragments prefixed with a star are called “wildcard segments”.

Wildcard segments can contain request format. For example, By requesting /users/foo/bar.json, params[:xyz] will equals foo/bar with request format of JSON. If you want to make the format segment must be exist, in config/routes.rb you can supply format: true like this:

get 'users/*xyz' => 'users#list', format: true

A route have more than one wildcard segments. Wildcard segments can be anywhere and can combine with other parameters. For example:

get '*abc/users/*xyz/:username' => 'users#list'

So, if client make GET request to /yuhuu/woohoo/users/foo/bar/example as the result params[:abc] equals yuhuu/wohoo, params[:xyz] equals foo/bar, and params[:username] equals example.