It is an old (and often politically incorrect) joke.
“How many [the butt of the joke] does it take to change a light bulb?”
The joke is used to exemplify failings in a particular group. The audience then laugh at the victim of the joke. Or so it used to be …
We will now attempt to bring the joke up to date.
“How many opticians does it take to supply a pair of spectacles?”
Answer: One to do an eye test and a prescription, then another to make a pair of spectacles (which are not right) then another visit for an eye test, and a check, then another visit to another branch to correct the mistakes of the previous optician [pause for breath] then a visit to yet another optician and an eye test, who makes a pair of spectacles, so bad that 5 seconds is enough to reject them [another pause for breath] then another eye test, then another pair of spectacles – which work, but the lenses are so badly finished, that the edges of the lenses are dangerously sharp, chipped, and too thick, then another go to produce another set of lenses that, if anything, are finished to such a crude standard that make the first set seem wonderful [pause for a deep breath] then reject lenses and take prescription to yet another optician who makes the glasses to said prescription (lenses much thinner and well finished), but they are not correct because previous optician made changes but did NOT put changes into prescription, so another eye test arranged, and more money laid aside for yet another attempt.
Somehow, I seem to have mislaid the humour in this joke. So, as a consolation prize, I offer another version of the joke which I consider to be one of the best …
How many people does it take to screw in a light bulb?
Back in the Golden Age, hundreds of servants would rush to screw in thousands of light bulbs, at my merest whim.
[Thank you, Londo Molari, Centari Ambassador to Babylon 5] I have the box set!
If, perhaps, you do not appreciate the joke, then, the next time, you can try screwing in your own light bulbs! I wish you luck!