The Window Experience Index (AKA the Windows System Assessment Tool - WinSAT) first appeared as a module in Windows Vista as Microsoft's attempt to create a base score for all PCs so buyers could better ascertain which systems would run the OS better than others. In theory, it was a good idea, but it didn't quite work out as well as planned.
Basically, the WinSAT portion of the module runs various tests on the processor, memory, 2D graphics - in the form of desktop performance for Windows Aero, 3D graphics for 3D business and gaming graphics and the primary hard drive. The numbers are then displayed, and the lowest of them then becomes the base score for the system tested.
The numbers generated differ from one version of Windows to the next. The initial Windows Vista release reported a scale of 1.0 to 5.9; Windows 7 went up to 7.9 and Windows 8.1 (which was pulled to begin with, but has since been re-instated with the installable WEI tool or available as a command line option) goes up to 9.9. The problem was, no one really knows what those numbers mean.
The scores were only ever as good as the hardware was at that time, so a perfect score across the board in Windows 7 would only score 5.9 in Vista. And a system that's many times more powerful would still score 7.9 in Windows 7, which ultimately gives no indication as to which is the better performer.
The idea behind WEI was to quickly determine where a bottleneck lay in your system. For example, if your 2D graphics section scored less than 3, then the Aero effects would be automatically turned off, and you could then go on to investigate ways to invest and improve the score until it peaked at its maximum. It's handy enough, but hardly a number that can be followed rigidly.
There were classic examples where an extremely low scoring WEI PC could easily beat a higher scoring PC when gaming or just in basic PC duties. The numbers involved were ambiguous at best and often resulted in many an argument in the collective forums.
Despite that, many people still insist on comparing the Windows Experience Index scores from one machine to another or compared to their older PCs. This is where the problem mainly lies with WEI, in that a reported score of 5.0 on the 3D gaming graphics to an upgraded score of 7.9 doesn't mean that The Witcher 3 is going to run at 1080 with every graphical option set to the highest. Granted, it doesn't state that anywhere in MS literature, but the feeling that a full 3D graphics score will run the latest game is very misleading.
To be fair, though, the Windows Experience Index isn't all that bad. It can help you determine, to a lesser degree, where you can spend your upgrade money, without getting too technical. But for serious use or to determine how the latest game will run on your system, you're better off with the likes of 3DMark or PCMark.