We're set to see the actual debut of Google Daydream on October 4, and it'll hopefully serve as a significant step up from Cardboard—their current virtual reality smartphone platform. If you want to get an early taste, you can set up a development kit—you just need two phones to do it.
One of those phones must be Google's Nexus 6P, their current high-end stock Android smartphone. The other serves as the "controller phone," and Google recommends a Nexus 5X (but that's not a requirement). You'll also need a cardboard headset, which are easy to come by in a more comfortable, plastic form. Google's support page has the full instructions to set things up, once you've wrangled all these ingredients.
The SDK allows developers to create VR applications for the Daydream platform, utilizing features like asynchronous reprojection, spatial sound, and (obviously) controlling the VR experience with extra hardware—in this case, an extra Android smartphone. Developers can work with Unity or UNREAL Engine 4, depending on preference, and make some cool VR experiences.
If you just want to mess around or actually create something for the October 4 launch, make yourself a dev kit.