What is the meaning of the Joke?
Hey, did you hear about the Jack who locked his keys in the car and he used coat hanger to get his family out?
Jack's family was inside the car. They could have opened the car door from the inside, so there was no need to use a coat hanger to force the door open.
This is a version of the kind of generic joke intended to make fun of people of a particular ethnicity, country, religion, appearance, behavior, or anything else you want to disparage.
For example, suppose you have a prejudice against people who drink tea. In this joke you would substitute "tea-drinker" for "Jack", to show how idiotic you think they are:
Q: How many [tea-drinkers] does it take to change a light bulb?
A: Three -- one to stand on a chair and hold the bulb and the other two to spin the chair.
I used to hear those kind of jokes more often when I was younger, but I'm not sure if I don't hear them now because society has lost tolerance for such things, or because it's juvenile humor that adults don't find very funny.