A snippet is a piece of HTML code, referred to as Content Block in the Snowfire UI. All available snippets pops up when you click the add button inside Snowfire. Each theme already got a couple by default (Text, Image, Video etc).
First make sure to enable developer mode by opening your Snowfire account, and click your name in the top right corner and choose Settings. Now hit the "enable developer tools" and head over to the my first snippet guide