A bit like the telphone- there really is no need to know how a hashtag works. All you need to know is that it does work!
The key is knowing how to use it. Wedding hashtags have become almost mandatory. So if you are having a wedding you need to come up with a hashtag which will be unique to you.
Commonly it is a combination of the couples name- perhaps with something added on. If Rebecca and Peter were getting married and their reception was at The Hyatt their hashtag could perhaps be- #petbechyatt.
When sharing any photos on social media (e.g. facebook) the person sharing the photo would include the hashtag #petbechyatt. Anyone clicking on this hashtag would then get to see any other photos that had been loaded on facebook by others.
The hashtag doesn't have to be derived from names- as long as it is 'unique' it will work well.
Photos are the most common thing to share with a hashtag but you can share any comments you have about the wedding as well. Join in and try it. It is nice for the married couple to be able to see all the photos taken by their family and friends.