what does # mean on twitter?

[LEX!Sツ]

if someone types in the word you have after the hashtag in the search box, your post will come up along with other peoples who hastagged the same word. for example, if they go:
people are annoying cry babies #just sayin
and someone else goes:
#just sayin, people need to grow up.
and you type in "just sayin" in the searchbox at the top, both of those posts will come up.