What is the meaning of " Pucker up "?

I'd like to know the meaning of this expression: Pucker up

By the way, I wrote two topics here and someone erased it, it was about how can I say "Por que será ?"
Because I saw in a text something like " Wht is it that you did it? " Por que será que você fez isso?

Please, don't erase my topic, it´s very useful, and I'd like to know the meaning of the expression " Pucker up " too!

Thanks!