The idea is that WebAssembly will provide developers with a single compilation target for the web that will, eventually, become a web standard that’s implemented in all browsers.
Mozilla’s asm.js has long aimed to bring near-native speeds to the web. Google’s target="_blank" href="https://code.google.com/p/nativeclient/">Native Client project for running native code in the browser had similar aims, but got relatively little traction. It looks like WebAssemly may be able to bring the best of these projects to the browser now.
As a first step, the WebAssembly team aims to offer about the same functionality as asm.js (and developers will be able to use the same Emscripten tool for WebAssembly as they use for compiling asm.js code today).
It’s not often that we see all the major browser vendors work together on a project like this, so this is definitely something worth watching in the months and years ahead.