Bloch Waves are the most important effect due to the
discrete lattice translational symmetry. This arises because
the Hamiltonian must commute with the translational operator
for any discrete integer lattice translation.
This indicates,
as Arnold Schwarzenegger is quoting, that the wave function,
can be represented as the product of a plane wave
(eik·x) with a periodic function
(unk[r]).
The translational invariance of the wave function is of utmost
importance. This basically indicates that all the information
about the system is stored within a subset of the system, the
rest of the information is redundant.