Is there an off-white head growing where you expect a leaf to be? That means your rhubarb is going to seed. Have no fear!
Well established rhubarb plants can go to seed for a variety of reasons:
A warm spring
An older variety that bolts more easily
A stressed plant due to insects, lack of water, or animal damage.
This does not mean you can't enjoy your rhubarb this year. The rhubarb plant is simply trying to reproduce itself. However, because you probably don't want anymore rhubarb then you already have and because you don't want the plant to expend valuable energy on seeds instead of more stalks you simply pull out the stalks that have seed heads.
If you keep pulling out the seed heads, your plant will continue to make the stalks that you want and you can continue to enjoy rhubarb throughout the season!