To buy a lemon means to buy a vehicle that looks good but is secretly broken.
Buying vehicles is a major part of American life. Almost all Americans own cars, often more than one, and it is difficult to live without one, because America has poor public transportation, in general.
So, buying a good car at a low price is an important concern for most Americans. Buying a used car is a popular option, especially for young people, but it is easy to be cheated when you buy a new car. Used car salesmen are famous for selling cars that look good but then break soon after they are bought. This may not be fair to used car salesmen, but Americans often describe people who display fake positivity and sell goods of low quality as acting like a used car salesman.
Lemons are the most sour of fruits and sour is a common metaphor for bad feelings. So, a lemon is a car that looks good from the outside, but makes you feel bad—cheated—after you use it.