  An unconventional insulating phase and a superconducting phase were recently
discovered in the twisted bilayer graphene [Y. Cao et al, Nature {\bf 556}, 80;
{\bf 556}, 43 (2018)], but the relevant low-energy electronic states have not
been clearly identified yet. In this work, I show that the interlayer hopping
induces a spatially modulated Dirac mass term in the continuum Hamiltonian, and
leads to a low-energy band formed by Dirac zero modes in the moir\'e
superlattice. This moir\'e band becomes extremely flat and thus strongly
correlated as the Dirac velocity vanishes at the magic angle, and enters a
quantum disordered Mott insulating phase at $1/4$ and $3/4$ filling, i.e., $\pm
2$ excess electrons per moir\'e supercell, which may account for the insulating
phase discovered in experiments.
