What do the # symbols and something after it mean on Twitter?

[Stru]

I see a lot of people tweeting things like "going to the gym #hardbody" or some sh*t like that. What does the "#something" mean and how do I put it and not sound like an idiot?

[hugefrog990]

It's like a subject or a trending thing. Like for instance you were doing something childish or something and you sarcastically said #bigkids

[Oliver Smith]

They are known as hash tags. You can search for all tweets including the same hash tag, so celebrities can ask people to use them, if a lot of people use the hash tag then it becomes more and more popular, etc.