This is such a great technique to use when you're trying to go for a more natural but still defined look. Here's a fantastic video by Beau Nelson, the creative director of Beaute Cosmetics, showing you how to achieve the look.
The picture on the top is my eyes without tightlining and the one on the bottom is them with it.
This is one of those techniques that works the best on people with light eyes as the definition is more noticeable. It's still a great thing to do for darker eye colours though as it gives the look of really subtle naturally thicker and darker lashes. This is also great for those who don't have a lot of lashes because it fills in the space between the lashes and gives the illusion of more lashes.