The standard cosmological model ΛCDM cannot respond to some important new results of modern cosmology. Challenges arise such as the Microwave Background Uniformity, the Hubble Tension, the El Gordo collision or impossible galaxies (z > 10) that the standard cosmological model does not solve. On the other hand, other models are proposed as alternatives. Among the models proposed in recent years, the Rh=ct universe is one of the most studied and it seems that it does solve these challenges favorably. Therefore, it is the obligation of every scientist to adequately substantiate the model and study the physical meaning it has through its equations. This model is based on the following equations, the restriction Rh = ct, where Rh is the gravitational horizon and the condition of zero active mass, (ρ+3p) = 0. We have carefully studied the foundation of these two equations and have obtained that the model responds to the condition of a universe of zero spatial curvature, being the only model compatible with it. Thus, to investigate the physical meaning of the models of universe with zero spatial curvature is to investigate the physical meaning of this model. To do so, we have obtained an equation that relates spatial curvature to the density of matter, resulting in zero curvature only being obtained if the density of matter is zero. Thus, we deduce that the physical meaning of models with strictly zero spatial curvature is an empty universe in all cases, highlighting the importance of spatial curvature.