It's all about dark roots that gradually fade to honey blonde tips but the result is really cool! Let your highlights grow and you got it girl (or at least this is how I did it!). It's very natural and model-ish, I love the effect. And you don't have to go to your hairstylist and pay a lot of money for it!
Even if you are not blonde, you can still make it as it works with any tone or textures - check Caroline Blomst, she's more like a red-head and she's got amazing hair.
I think the first one was Erin Wasson quite a long time ago and now it's everywhere! Just remember that, even it is a low maintainance trend, since you're going to light your tips so much, they'll need more moisture and you'll have to take care of them (to avoid they get dry) investing in some restorative products.