The term "dad joke" can refer to almost any punny joke, but it generally implies a one-liner pun that is dropped into an otherwise ordinary conversation in a way that is unexpected and often somewhat jarring.
As the name implies, this kind of humour is often inflicted by fathers on their children, generally to the great embarrassment of the kids (the more so, the better), although the term 'dad jokes' is also used more widely to describe the same kind of jokes being told to anyone.
See also: