Original: The continuum concept ignores the fact that matter is made of atoms, is not continuous, and that it commonly has some sort of heterogeneous microstructure, allowing the approximation of physical quantities, such as energy and momentum, at the infinitesimal limit.
Simplified: The fact that all matter is made of atoms and that it commonly has some sort of heterogeneous microstructure is ignored in the simplifying approximation used in continuum mechanics.