Native Client is a tool that Google is developing which allows programs written for x86 architecture CPUs (all of the PCs from Intel and AMD) within any Internet browser.
What does this mean? Basically that Internet browsers can run programs much more complex with more direct access to the CPU and hardware of the PC. Much closer to the functions of the browsers to meet the operating systems.
Native Client frees running programs within a browser ,making them to have different versions for different operating systems(tasks).
While Native Client is at a very early period of its development, it is now available for testing on Linux, Windows and Mac and to get an idea of what is allowed here ,like an image of Quake running inside an Internet browser with the help Native Client.
video illustration of Native Client running some programs.