Saving your seach results from a swarm of marauding Os is the search giant's latest mission for its army of users.
Playing along with the latest Google Easter Egg, you enter the phrase "zerg rush" at the search giant's home page. In return the usual search results pop up. Then slowly yet surely a group of invading Os (representing the Os in Google) will swoop in from above, below, and the sides, intent on eradicating your search results.
Your only weapon is your trusty mouse (or your trusty finger if you're using a touch screen). Clicking or tapping on the attacking Os will eliminate them one by one. Google will keep track of your hit count, showing you how many Os you've defeated and your average rate per minute.
Ultimately, though, you're fighting a losing battle, since the Os are just too strong and heavy in number. Finally, as your last search result is vanquished, two Gs (which stand for Good Game) appear on the screen with your total score at the top. If you manage to defeat a sizeable number of Os, you can post your results to Google+, where other users have been savoring their victories.
The term "Zerg rush" is a tactic from the classic game Starcraft.
Those on the receiving end of such an attack had to defend themselves against a rush of invading Zerglings by quickly taking each one down even though defeat was usually imminent.