The concept is illustrated in the .gif below:
Assume that all three colors enter the turn with the same speed. Normal logic would dictate that taking the inside would be the fastest route, as it allows the driver to travel the least amount of distance. While this is true, the tight angle also restricts the car from reaching the highest speeds possible. (This is what racers call “straightening a turn”.)
So, wouldn’t the blue line be ideal?
Not quite. While the outside line enables the car to max out it’s speed, the added distance traveled makes it slower than a mid point between the two.
Wiki has a nice blurb with pictures on the ideal line, and even goes a little further with points on an early or late apex and their pros and cons.