As explained in the book, Dr. Kasner's 9-year-old nephew, Milton Sirotta, came up with the word googol when asked to think of a name for 1 followed by 100 zeros.
A googol is the large number 10100. In decimal notation, it is written as the digit 1 followed by one hundred zeroes: 10, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000.