As crazy and absurd as it sounds, yes this does do exactly as the title says. Knock is a new app for both iPhone and Mac that lets you unlock your Mac by simply knocking on your iPhone.
Using low energy Bluetooth 4.0, the iPhone and Mac pair up, when Knock is running on both devices, you simply knock on your phone to unlock your Mac. No having to type in your password.
It does take a bit of effort to set up but once you get going it works brilliantly and is actually faster and safer than typing your password - especially out in public.
To demonstrate how easy it is, they've also produced a pretty funny promo vid for it.