Someone please explain it for me( CRYPTOGRAPHY)?

There are two points in the paragraph. Firstly, when they say that a change in one bit of plaintext affects all 64 output ciphertext, it means that the cipher scrambles the plaintext very thoroughly, such that even a small change in the plaintext will produce a huge change in the ciphertext. Such a property (sometimes called "avalanche") is very important for a strong cipher. If otherwise a small change in plaintext produced a small change in ciphertext, it would make it easy for a code-breaker to derive simple equations involving known plaintext, ciphertext and unknown key bits to solve for the key bits, thereby breaking the cipher. The second point just says that the 64-bit block cipher is essentially just a big-ol' substitution cipher.