Many encryption algorithms depend on the use of a random-number generator. If my computer and your computer both generate identical random-number sequences given the same seed, then we can build a reasonably secure cryptosystem, or so it would seem. As long as the sequence is truly random with no repeating patterns, there are no patterns for the codebreaker to observe. The book points out the problem with these schemes. Give them enough cycles, and they will repeat themselves.
Repetitions produce patterns, on which a cryptanalyst thrives.
Novice cryptographers might assume that their security lies in the secrecy of the algorithm itself. You might reason that a codebreaker can have the ciphertext message and even the key and still not be able to decrypt the message because the algorithm itself is unknown. For example, a file compressed with a Huffman tree appears to be a random pattern of bytes, not even the same length as the original plaintext. It looks pretty secure to the casual observer. There are flaws in this logic.