It is a real string, not a sample
Each pluck fires a burst of noise into a delay line that feeds back on itself, filtered a little more on every pass. That is how a plucked string decays in the physical world, and it is why two strings hit a fraction apart sound like a chord rather than two sounds at once.
Drag across the strings and you will hear it. Fast strums buzz, slow ones separate into individual notes.
What to press
Mouse across the strings, or A S D F G H for the six of them, low to high. The chord buttons hit the right strings in the right order with a small delay between each, which is the difference between a chord and a noise.
Standard tuning, low E to high E, no capo. Anything you know on a real guitar works here.
Recording
Press record, play something short, press it again, then play back. It keeps the timing, so a sloppy strum plays back exactly as sloppy as you made it. The original doodle did this too and let you share the result, which is why people spent a very long day with it. The other thing worth strumming at is the swarm in zerg rush, which wants the same fast fingers for a completely different reason.
The doodle
It went up in June 2011 for the 96th birthday of the man behind the solid-body electric guitar, and it stayed an extra day because nobody would stop playing with it. Guitar is also a word people cannot spell, so google gitar, google gutar, google guiter, google gultar and google gutair all end up looking for the same thing.
If you want the other doodle that survived, the maze at google pac-man is still there.
No sound?
Click a string first. Browsers refuse to make noise until you have interacted with the page, and a muted tab will stay silent no matter what you press.
