A number equal to ten to the power of a googol. That is, 10^10^100.
If you wanted to see an example of a googolplex, you would have to look at the entirety of the universe on a subatomic level: the estimated number of atoms in the universe is around 10^10^78.
If you tried to print all of the zeros in a googolplex in 1 point font, the result would be a line that could stretch from one side of the visible universe to the other (60-80 billion light years according to current observation and theory) and possibly make the trip around at least once more, assuming that there was enough matter in the universe for that process to take place.
And the googolplex is not the last number that we have assigned "googol" or "plex" to.
Any example I could give wouldn't mean anything to you because of the sheer scales I would be talking about.
For an example of a googolplex, do some research on quantum theory, the quantum inflation, and multiverse theory.