That is explained to be emptiness.
That, being a dependent designation,
Dependent designation is one of the 'three dependencies' asserted by the Madhaymikas, the others being dependent arising, which relates to the concept that all existents arise from causes; and dependence upon parts, the idea that all existents are composite. Each of these dependencies are used, separately and together, to help establish an understanding of sunyata, the absence of inherent existence, which is the third of the three marks of existence.
Phenomena are merely imputed by terms and conceptuality in dependence upon their basis of imputation.