Just a quick explanation:
This is a ghosting test. Ghosting is a problem some keyboards face, where certain combinations of keys don't all register at the same time.
Holding both shifts then typing a pangram (a sentence that has all letters of the alphabet), you can find ghosting problems on you keyboard. IF you have ghosting problems it doesn't mean your keyboard is bad, more so that it isn't 100% anti-ghosting, which is never needed anywhere on the face of the Earth ever. (Other than for e-peen).