I believe that what is being referred to here are molecules with no dipoles. This is where there is no overall separation of charge (a dipole moment) across the molecule.

Read literally the previous definition requires the atoms in the molecule to all be identical (no two elements have exactly the same electonegativity). For molecular elements (non-metallic) these consist normally of just two atoms.

However as long as the resulting molecule has no overall separation of charge (normally due to symmetry) then you can have non-polar molecules from a combination of different atoms (Carbon Dioxide and Methane spring to mind). The individual bonds in these are however polarised.