If you don't know what Pokémon are (or, if we're talking about the ongoing phenomenon, is) this isn't the place to find out. But I'll do my best, since you'll need to know the basics now that Google's unleashed the little critters on the world for a day. Just a day, as far as we know, but I assume nothing in a world where Google and Pokémon publisher Nintendo suddenly join hands.
Grab your iOS or Android device: they're the only way to see what I'm talking about. Bring up Google Maps (or download Google Maps if you don't have it -- see what's happening here?) and tap the "Search" bar. Notice that little thing that looks like the CBS logo, only light blue?
Tap it and you're off, the screen swinging across the country to Mountain View, California. That's Google HQ, a.k.a. Google Campus, a.k.a. a bunch of Google buildings with numbers like "2000" and "1900" and "1950." Pitched between the latter three you'll notice a tiny house with a chimney. That's Google's new Pokémon lab, originally located on Cinnabar Island, according to some all-official-sounding Pokémon compendium called the Bulbapedia.
The idea behind Pokémon is that a bunch of cute little fantasy pets smack each other around, but first you have to catch them, and that's where Google Maps comes in: beside the Pokémon lab, you probably noticed a few in the wild. Tap them to catch them, simple as that, after which you can take a closer look in Google Maps' new inline Pokédex (basically a Pokémon Rolodex).
You can go for all 150 yourself, scattered around the planet, or skip the hunt and tap the Pokémon April Fools' wikia, which catalogs the lot, plus one more. That "one more" allegedly only appears once you've found the other 150, and then randomly across a potential set of locations.
Oh, and if you do catch 'em all, Google says you'll be in the running for a new job with the company: Pokémon Master. You probably think I'm kidding, don't you.