That just initializes the sum to zero. Since sum is a float, vart used a float value to initialize it. Numbers with decimals are doubles in C++, but since you are using floats, you have to add the f to the end to indicate that you want to use float. (doubles are actually recommended over float assuming you don't have a requirement to use float.)
We were required to use float. You guys are the greatest. I changed just a few things to get it to look like what I have actually learned but nonetheless, thank you so much. Now I get to move on to step 4 which should be interesting. Choosing every third integer to sum.