It's one of those things that you really only understand if you've experienced it. It's when one person finds another that is perfect for them and for whatever reason they can't be with that person. Usually they have some sort of very close relationship over a long period of time with the object of their affection, but the love is true and real and strong and inherently tragic because it is and always will be unrequited. Often abbreviated "TL."
Kristin Cavalleri is Brody Jenner's tragic love.
Stephen Colletti is Lauren Conrad's tragic love.
Carrie Bradshaw is Aiden Shaw's tragic love.