A word or phrase preceded by the symbol # that classifies or categorizes the accompanying text

But what does that mean in practice?

Hashtags are words or multi-word phrases that categorize content and track topics on Twitter, Facebook, Instagram, Pinterest, and other social media outlets. Hashtags are preceded by the # symbol. Two examples are #picoftheday and #sweepstakes.

People can use hashtags to search for posts with a specific theme, helping them find posts and tweets that interest them. On the other hand, if you want people to find your own posts, they act as a signpost for other readers, too.