A snippet in search engine terms is the individual result Google displays for each page in a search.
A rich snippet, is snippet that contains additional information that improves the result, helping the user to gain more information about the result.
This extra information is usually generated using structured data, or as it’s also known, schema markup.
Using schema markup, your snippets will be of a better quality and are more likely to stimulate a click.
The example above shows a snippet without structured data applied, and the bottom one with the structured data.
Clearly the bottom snippet is more likely to get clicks as it provides the user with more information.