Quick answer it means the number increasing by 100% will double (multiply the number by 2).
Long answer:
To find a percent of something is simple, take the percent you want to find and turn it into a decimal equivalent then multiply it by the number.
In the case of 100%, with a number "n."
100/100 = 1
1 * n = n
You then add the value you got out of taking the percent of the number to your original number so... it looks like this:
n + n = 100% of n.
n + n is the same as 2*n. (Multiplication is just a easier way to add... so 2* something is just that thing added twice)
