The IV for CBC mode of many cyphers is 8 octets (64 bits) of random data which is prepended to the plaintext before encyphering. The effect is to scramble the cyphertext still further. It is rather like the salt in builtin crypt digesting.

Use of IV is mandatory for some CBC protocols. See, for instance, RFC-2405, The ESP DES-CBC Cipher Algorithm With Explicit IV.

After Compline,
Zaxo