"Null" comes from the Latin word nūllus, meaning not any. It is a word that means without value, amounting to nothing, nil, lacking, nonexistent; zero. In cryptology it is something that is useless when it comes to decoding the message, and is often times used to throw others off the right track to decyphering the message. It can be pretty much anything, but is most often random letters, or sometimes words.
Ex. Monoalphabetic substitution/shift cypher:
XFBS FPGG UPTF FUIF XJAB SEXY
This is split into sections of four letters each so word length won't give it away. It's actually quite easy: To get the plaintext (the message intended for the recipient), just take the letter in the alphabet before the cyphertext (the encoded message). So A = Z, B = A, C = B, &c.
The null letters are X and Y. It works out well that the last four letters actually spell a word, because that can also throw people off the real message.
A null cypher is a cypher where the plaintext is hidden in a lot of material unrelated to the message. It is not only a cypher, but a form of steganography as well.
Steganography is, in essence, the art and study of hidden messages. Whereas cyphers are obviously secret messages (DPXT), steganographic messages are hidden, either as a cypher or not (Catherine objects whatever's said), and should not be obvious. Invisible ink is a good example: It's invisible, -- hidden. The message written in invisible ink can then be plaintext or cyphertext. (COWS)
Acrostics, in poetry, is where each line's first letter spells out a word or message. This is one of the most common forms of null cyphers. In poetry, often the message is intended to be known, but, of course, for a null cypher that intention is reversed.
You can, of course, mix it up a bit and take the second, third, last, &c. letter of every word, or take certain letters at specific distances apart to try and hide your message better. If no one but the recipient knows to take the first letter of the first word, the fifth letter of the second, the second of the third, and the fourth of the fourth, &c. then your message is pretty well hidden.
This method of encryption is called acrostic cypher (which makes too much sense, if you ask me). It was featured in JJ Abrams's television show "Alias," but was called "skip sequence cyphertext" (season two, episode nineteen: Endgame). Aside from modern uses, it became quite popular in the Renaissance period of Europe and was used in many forms of literature.