Original: The prosencephalon, the mesencephalon (midbrain), and rhombencephalon (hindbrain) are the three primary portions of the brain during early development of the central nervous system.
Simplified: The prosencephalon (or forebrain), the mesencephalon (midbrain), and rhombencephalon (hindbrain) are the three primary portions of the brain of vertebrates. They develop very early during development of the central nervous system.