Snippets make it possible to render markup from a source template anywhere, regardless of the context and the usual rendering order.
This is particularly useful for assigning dynamic values to e.g. Facebook's Open Graph metadata tag; normally you would not have access to e.g. product information in the master template, but by defining a snippet in the product details template and using RenderSnippet() in the master template you do.
Define a snippet source with SnippetStart and SnippetEnd tags.
Select the code snippet in your template like this:
Insert this tag where the snippet should be rendered (same or another template):