To store data, SQL Server uses pages that are 8 kb data blocks. The amount of data filling the pages is called the fill factor, and the higher the fill factor, the more full the 8 kb page is. A higher fill factor means fewer pages will be required resulting in less IO/CPU/RAM usage. At this point, you might want to set all your indexes to 100% fill factor; however, here is the gotcha: Once the pages fill up and a value comes in that fits within a filled-up index range, then SQL Server will make room in an index by doing a "page split."
In essence, SQL Server takes the full page and splits it into two separate pages, which have substantially more room at that point. You can account for this issue by setting a fill-factor of 70% or so. This allows 30% free space for incoming values. The problem with this approach is that you continually have to "re-index" the index so that it maintains a free space percentage of 30%.