When on twitter, why do people put a # at the end of something? What does that even mean?

[Lilyanna Keshmiri]

I've never had twitter and my family and I saw this commercial with that girl from modern family, for the pepsi commercial, and she twittered everyone that that one soccer player was there. And she put a # at his name, what is that?

[Ben]

It's called a hashtag. You put it on words that say the topic of the tweet. That way, if you want to search for tweets about something specific, you can just search for #that_thing and you won't get tweets that mention the word in passing, only ones actually relevant to you.