In the equation GPP - R = NPP

GPP is Gross Primary Productivity

NPP is Net Primary Productivity

R here is :

Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).

$$\begin{gathered} \mathrm{GPP} = \mathrm{NPP} + \mathrm{R} \\ \mathrm{or} \\ \mathrm{GPP}\hspace{0.17em}- \mathrm{R} = \mathrm{NPP} \end{gathered}$$

The Net primary productivity (NPP) is the available biomass for the consumption to heterotroph (herbivores and decomposers).