Googolplex is a really large number made up of
googol and
plex.
googol is a made up word to mean 10 to the power of 100 or 1 with 100 zeroes after it.
Plex is 'plus exponent' which means 'to the power of a positive number'. (the alternative is minex, 'minus exponent' which is 'to the power of a minus number')
so
googolplex is 10 to the power of a googol, or 1 with a googol zeroes after it.