That is the flower head of the plant. Your rhubarb has bolted.

It's going to flower and set seed, very pretty though. More details on the site below, for reasons and what to do.
http://www.gardeningknowhow.com/edible/vegetables/rhubarb/rhubarb-bolting.htm

It's a seed stalk.
Remove seed stalks as soon as they appear. :o))

Thank you everyone !

Never having seen pink rhubarb flowers before I looked on google. No info on pink flowers but lots on why they flower and how to discourage it. Seems a lot can depend on the variety as well as growing conditions.