View that display single node which has id the same as last part of url (from last / to the end)
Normally the parameter is not named, it is a single % without name:

portfolio-entries/%


The example node/%/feed is also a single % in the middle part of the url. The third part of the url is a fixed feed, so that only urls with a /feed at the end will match. 

To use this parameter for filtering you need to set a contextual filter in the advanced settings. If you only have one parameter, this filter will automatically get this, when selecting the field you want to filter.