what does this mean on twitter?

[Marley D]

It has the persons name who posted two arrows that kinda make a box and then by and someones name.


Hope you can understand

[Jackie]

It means the first person just "re-tweek" the second person tweek. In other word, the first person just copy the exact tweek from the second person and post it up so that their followers can see. You do this when you find that someone you are following just tweet something meaningful and you want to share it with your followers.
hope this answer your question.