One Billion is 1.0E9, or a one followed by nine zeroes. so 6.0E45 is one billion to the power of 6, or six billion-billion-billion-billion-billion-billion, times six. At that point you can kind of just mad-libs each billion to be whatever you want "A billion Earths mining for a billion days, duplicated a billion times in a billion different simulated universes, and the odds of finding a hash after all that is still one in a billion, squared."

Or you can plug it into Wolfram Alpha and get it to find a comparison, in this case 6E45 is 0.011% of all possible chess combinations.

It does get worse than that though. A project I played with last year had me asking how many unique permutations there were for a 1920*1080 monitor. That number was around 10^10^77. I don't have any formal maths training and that one really threw me for a loop. To summarize 10^x means you have a number that's x digits long. So 10^6 means I have a six digit number. 10^10^x means that the number that describes how long my number is is x digits long. so 10^10^77 means that if I wanted to write out my number and count the number of zeros in it, that counting would give me a number 77 digits long. Math is weird.