A hash tag is any word beginning with the pound symbol and people use hashtag to reference specific topics and clicking on one will take you to search results. Hashtags are way to explore concepts, finding information and increase the reach of your tweet, many conversation on twitter involve a hashtag especially those that are meant for a large audience. It's a unique way of categorizing the content per se.
hashtags are really just a unique way to exploring thoughts in potentially real time.
By applying a hashtag, you are essentialy linking your tweet to a particular term, this might be specific to a brand, a marketing campaign, an event or even a location. Hashtags increase the discoverability of content on twitter, when someone searches for a particular term or comes across a hashtag that is of interest, they can click on it. When apple hosted their press event watching the iphone6 applewatch gathered to share their reactions under a common hashtag, when there was a technical glitch people were able to view the tweets around that hashtag to identify that the problem was not unique to them, it was actually widespread. as a business, you can take advantage of hashtags, you can decide to create conversations around hashtags, by hosting a contest, watching a special campaign or asking your customers to share testimonials using your brand