(If you’re puzzled by the odd symbols in the cartoon — Dan Piraro says there are 2 in this strip — see this Page.)
To understand this cartoon at all, you need to recognize the figure at the door as Mr. Potato Head, and you need to know the following fact about this character, from #2 in a posting of mine on 5/11/14, where I quote Wikipedia saying that Mr. Potato Head is
a plastic model of a potato which can be decorated with a variety of plastic parts that can attach to the main body. These parts usually include ears, eyes, shoes, a hat, a nose, and a mouth.
Ok, plastic body parts. But there are some not on that list.
Then you need to add in Mr. Potato Head’s reference to his urologist. On urology, from Wikipedia:
Urology (from Greek οὖρον ouron “urine” and -λογία -logia “study of”) … is the branch of medicine that focuses on surgical and medical diseases of the male and female urinary tract system and the male reproductive organs.
So what does Mr. Potato Head have in his hand, for his trip to the urologist? Plastic genitals, penis and testicles, for the specialist to check out. It’s so much easier when everything is detachable.