I was confused as well, but it does appear to be your anticipated annual increase in contributions. Since I already max out my retirement contributions, I entered "0" and it showed me how long it would take me to reach my goal if I make the same contributions each year, which is what I wanted to see.

If you're not maxing out your retirement contributions, entering an "indexing" number will show you how much faster you can reach your goal by increasing your annual contributions, as opposed to if you didn't increase your annual contributions.