Padding is a term used in cryptography. Very often, messages (to be encrypted) start or end in a well-known way (Dear Bob, ..., Yours, Alice). This is a problem, because that knowledge could be used to break (or start to break) encryption. To prevent this, a number of random characters are added at the beginning or the end of the message.
This procedure may also be necessary because many cryptographic methods operate on blocks of code. If a block is not full, additional characters need to be filled in. Filling in random text has proven very useful.