Mass Concrete is defined as:
“Any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of the cement and attendant volume change to minimize cracking.” (ACI Manual of Concrete Practice).
In simple, that volume of concrete is called mass concrete which generates so much heat and change the volume so much that you have to take some measures to minimize the resulting cracking.
When dimensions are > 1m or 3ft, temperature rise should be considered. (Panarese)
Common thought is that mass concrete principles only apply to large dams. But they apply to any large pour e.g. massive foundations, bridge piers, thick slabs, nuclear plants, structural columns, etc