A car is typically called high mileage if it has put more miles on it than any average car with similar age. The average driver puts on 10,000 to 12,000 miles per year on a car.
Any miles above that are generally considered High Mileage vehicles.
So if a car is 5 years old and has 100,000 miles it would be considered high mileage for the year.
Although cars with a lot of miles in them are more likely to have issues than similar cars with lower miles, often you can get a good deal on used cars with higher miles.