Unity Web Player is a plug-in for your browser that allows you to play games developed with the Unity engine.
You've probably got Flash installed, which allows you to see some pretty cool things from your browser, but the Unity Web Player produces really impressive results, with excellent smooth running 3D graphics usually seen only in standalone programs.
Unity itself is a Mac development tool for developing 3D games on all platforms, and while that means there are some Apple only releases, there are a lot for Windows too, and all of the browser games are cross platform.
Maybe the worst thing about Unity Web Player is that the high end graphics mean you'll need a pretty fast machine and a good internet connection to load up the games quickly. However, assuming you have this, the plug-in is highly recommended as you won't have seen games that look this good running on a web page before.
Try Unity Web Player to play some super cool online 3D games.