What does # mean on twitter?

[Danielle]

I know it's hash tag, but what does it mean, and is there specific words that go after it or do you make them up?

[Caskett]

like if you saw something disgusting, you would do #ewwww, #gross, etc.

[Sydηəy]

It's kinda like the subject.
You can make up whatever you want but there are certain ones that a lot of people use, like:
#oomf - one of my followers

and others you can check the trending topics