Here's an excerpt from the developer:
"Your brain can sometimes do funny things to letters. OpenDyslexic tries to help prevent some of these things from happening. Letters have heavy weighted bottoms to add a kind of "gravity" to each letter, helping to keep your brain from rotating them around in ways that can make them look like other letters. Consistently weighted bottoms can also help reinforce the line of text. The unique shapes of each letter can help prevent flipping and swapping."