Community snippets are small pieces of code, packages, or utillities you can use with umbraco.
Anything can be created as a snippet. Do you have some fantastic xslt, then post it. Maybe a c# snippet for working with the api, well post it. Or maybe you created a small script to answer a question on the form? Well create it as a snippet.
Just write whatever you want to post in a .txt file, or find an existing file and upload it, and *presto* it will be available to the entire community.
The snippet listings are created and maintained by the community, for the community.