From the Internet Explorer Team’s Blog:
The Internet Explorer Platform Preview is a light-weight frame around the core IE platform which includes the rendering and layout, object model, parsing, and script engines. It’s a way to try out the platform, and the experience improvements we’re making to performance, standards support and interoperability, enabling “the same markup” to work.
We designed the Platform Preview to be installed, side by side with IE8. The Platform Preview is not a replacement for your daily browser; for example, the Preview does not have an address bar, a back button, anti-phishing or malware protection. The Platform Preview does include debugging and diagnostics tools for developers. It also includes a way to provide feedback.
For more details on what you can do with Internet Explorer 9’s Platform Preview, read through the User Guide they’ve provided and if you’re a web developer, be sure to check out the Developer’s Guide to see what’s coming with Internet Explorer 9.