Superplasticizers are chemical admixtures added to concrete to make it far more fluid without adding extra water. They are formally called high-range water-reducing admixtures (HRWRs) and are classified as Type F (high-range water-reducing) or Type G (high-range water-reducing and retarding) under the ASTM C494 standard.
To qualify, an HRWR must cut a mix’s water content by at least 12% compared with a control mix, and it can cut it by more than 30%. Less water means a lower water-cement ratio, which yields stronger, less permeable concrete, or the same mix can be made highly flowable for tightly reinforced forms and self-consolidating concrete. Older superplasticizers were melamine- or naphthalene-based; most modern products are polycarboxylates, which hold workability longer.
Source: National Precast Concrete Association: Water-Reducing and Set-Controlling Admixtures
Image: Concrete pouring at Spangdahlem Air Base, U.S. Air Force photo by Airman Sydney Franklin, public domain, via Wikimedia Commons.