What does the term "Slump" stand for?
Slump is a measure of the consistency of concrete, or how "wet" or "dry" the mix is. The lower the Slump, the dryer the mix. 

What are "Superplasticizers" used for?
Superplasticizers are used to further reduce water demand over water reducing admixtures. It is generally used to produce high strength concrete or to improve the workability of the mix.