Using a hashtag on Instagram is just as easy as on any other platform and works just like Twitter.

just like Facebook you need to have your account set to public otherwise your hash tags will not show up in your chosen topic search.

When you have written your new post and it's time to insert your hash tag, instagram tells you how many other searches are taking place via trending figures:

#minecraft search:

If you want to search for a topic then simply type #minecraft into the search bar and the same search should come up. When you click on one then a stream of images connected with the chosen hashtag will load on the new page.

it's that simple. I do believe the most simplified way of describing hash tags is the same as keywords on your websites the you want to target a specific audience.