Why does # appear on Facebook pictures with words sometimes?

[MellowYellow88]

Why does a picture post with #smiley girl # long hair don't care #bad attitude etc. these are just examples

[Leon]

Based on twitter hash tagging, a way to trend on twitter using the same set of words. On Facebook it has no purpose but to make you look like an idiot

[Cherrie]

Their Facebook account is probably linked with their other social media accounts such as twitter or Instagram and when they post it on those platforms (where # is used) Facebook automatically uploads those photos they posted with the same caption as well.

[msid9999]

It's called a hashtag. It is used after posts on social media sites mostly to kind of describe the situation in the post. Or it could be something related to it.
So if I said,
'God, I'm so mad right now. Life is totally unfair.'
I'd add #LifeSucks or #Angry
Another example
'Anybody up for chicken wings?'
#ReadyToEat #Hungry

Hope I explained this well!