A pun is a play on words that uses a word's multiple meanings, or the close sound of two different words, to create humor, irony, or a sharper point. Poets use puns to make a line work on more than one level at once, often giving a phrase a light surface meaning and a second, more pointed one underneath.
Example
I asked the moon to keep my secrets safe;
it gave me a quiet light and left me in the dark.
[n] a humorous play on words; "I do it for the pun of it"; "his constant punning irritated her"
[v] make a play on words; "Japanese like to pun--their language is well suited to punning"