a product that is a "lemon" is a faulty product, one with defects, one that doesn't work properly. Most often used to talk about cars.

People will describe their cars as "a real lemon" when it needs to be repaired often. I had a car like that! It needed repairs far more often than other cars that were the same make and model. Everything seemed to need fixing. Within the first year, the brakes needed adjusting. The wheels needed an alignment. The transmission didn't work properly! It was a real lemon!

A lemon means that the car was broken when you bought it.

The answers are very interesting. This is not a phrase used in British English!

google "what is a lemon car?" https://en.wikipedia.org/wiki/Lemon_(automobile) In US parlance, a lemon is a vehicle (often new) that turns out to have several manufacturing defects affecting its safety, value or utility. Any vehicle with such severe issues may be termed a lemon and, by extension, so may any product with flaws too great or severe to serve its purpose.