What is meant by "pun intended"?

Pun intended means that the joke was a deliberate one.

A pun is a play with a word, e.g. "You can tune a guitar, but you can't tuna fish."

Sometimes authors accidentally do this; that is, they did not mean to play with a word, but a reader can interpret the author's work as a pun.

However, here, it is specifically stating that this pun was intended, or deliberate.

I'm surprised that nobody has mentioned that the phrase "pun intended" is a form of wordplay itself:

As others have commented, a pun is a play on words, usually using words that sound the same or similar to give multiple meanings to a phrase.

Sometimes when writing or speaking it is hard or unavoidable to use a phrase that creates such a double meaning - potentially obscuring the speaker's point and/or making it seem like they are joking. In this situation it is common for people to say

(No pun intended)

This, I believe, is the original phrase.

Playing on this, some people occasionally reverse the phrase by removing the negative:

pun intended

to make it clear to the listener that yes, they are using wordplay to make their point.

(Interestingly Google has ~11 million hits for "pun intended", but ~21 million for no pun intended, even though every page that contains "no pun intended" must contain "pun intended"!)

"Pun intended" literally means "There is a pun (a play on words) in this sentence, and I put it in intentionally." This distinguishes it from unintentional puns, where the writer didn't mean to put it whatever double-meaning is there.

Unfortunately the example sentence you give doesn't have enough context for us to tell what the pun is; I would guess that either "wiring" or "differences" is used in a slightly different way in the previous sentences.

Pun

The pun, also called paronomasia, is a form of word play which suggests two or more meanings, by exploiting multiple meanings of words, or of similar-sounding words, for an intended humorous or rhetorical effect.

Pun intended probably is used to indicate that the ambiguous phrase was intentional.