It means that the person has used some words which produce some kind of word joke, but they didn't mean to make a joke. Usually a pun is a kind of play on words.
So sometimes people mean to make a pun, but other times they say something and they don't mean to make a word joke, so they say so, to make it clear.
For example the words son/sun are pronounced the same so we can make a joke using those.
There are many other words in English which can be used to make a pun or word joke.
For example if someone makes a pun I could answer "Very punny" which would be making a joke with the word "pun" that sounds like "fun". Instead of saying "funny" I say "punny".
More samples http://ask4essay.com/thread/what-does-no-pun-in...