A googol is a famously huge number, literally ten to the one hundredth power. A math teacher might offer extra credit to students who correctly write out the digits of a googol on the blackboard.
If you want to write out a googol, write the number one followed by one hundred zeros. Mathematicians sometimes use the idea of a googol to illustrate the difference between an enormous number — one that's too large to imagine — and infinity. The word googol was invented by the young nephew of mathematician Edward Kasner, one of the authors of the 1940 book "Mathematics and the Imagination."