The optimal coding referred to here is called Huffman coding.
Huffman gave an algorithm which produces a prefix code which is
optimal among character codes: Huffman coding uses a specific
method for choosing the representation for each symbol,
resulting in a prefix code (sometimes called "prefix-free
codes", that is, the bit string representing some particular
symbol is never a prefix of the bit string representing any
other symbol) that expresses the most common source symbols
using shorter strings of bits than are used for less common
source symbols. Huffman was able to design the most efficient
compression method of this type: no other mapping of individual
source symbols to unique strings of bits will produce a smaller
average output size when the actual symbol frequencies agree
with those used to create the code. The Wikipedia article has
complete details. Huffman's original paper, " Method for the
Construction of Minimum-Redundancy Codes", contains a proof of
optimality.