When you discover you're disappointed with the car you just bought, this is the amount of money you can expect to recover when reselling it.
If you borrowed money to buy this car, cars with low resale values can put you "Upside down," meaning you will owe more for the car than what you can expect to get for it. Cars with a high resale value will allow you to pay down your remaining loan.
Generally speaking, one doesn't normally need to worry about resale value unless they bought a car because they believed it would suddenly make them "cool."
"I bought a Honda Civic because I thought it would make me cool; but that didn't work. Luckily, it has a high resale value, so I can get rid of it and buy something better."