What is mileage? And is it better to buy a used car with higher or lower mileage?

I'm looking to buy my first car in the next year or two, and I always hear my parents talking about how this car has 12,000 miles on it and this car has 50,000 miles on it. So when buying a used car, what is better to buy, a car with high mileage, or a car with low mileage? And what is mileage?

____________________
buy used car|used car for sale