What that means is that on the row where you are supposed to cast on 10, you will be adding 10 stitches to that row. You will then knit the remaining 15 stitches, so by the end of the row, you will have 25 stitches on the needle.
The cable cast on is a good one to use for this purpose. You can find the video that shows how to do it here:
(Scroll down the page.)