This hashtag is made to be used in combination with any other hashtag, in order to introduce that other hashtag.
Use it only once per hashtag, and use it in a post that only contains the hashtag you're introducing along with #intro. Because if you use #intro in a post with multiple hashtags in it, then that post will come up whenever anyone searches for the intro to any of those other hashtags, as well. Which is just, plain, disorganized.
This #intro tag allows users to quickly get caught up on what a certain hashtag means, and how it is used.
The purpose of #intro is a bit redundant to the ~wiki, but it's a quick and easy way to see how a hashtag was originally introduced, whereas the ~wiki always contains the most up-to-date meaning of the tag (unless you go check the first version of the wiki article).
So, for example, if you want to see how "#hashtag" is introduced on Openbook, you can just search "#hashtag #intro".
See Also: hashtag_combos