This is a feature that appears to have been around for a long time (at least back to 6.x), that I've never really noticed before, but seems really useful, and so worth a quick post about it!
Often when writing a blog there will be a standard piece of text, or link that you use in many blog posts, and end up copying and pasting between posts. This can become tedious and if you need to update the text is a lot of effort, this is where snippets can help. They allow a keyword to be assigned to a piece of text for a blog, that can then be used in any post for that blog. Any updates made to the text will automatically be applied to all the posts using the snippet.
You can see how to setup snippets in the Telligent Documentation (9.x, 8.x, 7.x, 6.x).
When writing a blog post you then just enter the name of your snippet surrounded by square brackets (this isn't immediately clear in the 9.x UI/docs), for example if the snippet was named "about", if you enter "[about]" it would be replaced by the text defined.