A "FILLFACTOR" is one of the important arguments that can be used while creating an index.

According to MSDN, FILLFACTOR specifies a percentage that indicates how much the Database Engine should fill each index page during index creation or rebuild. Fill-factor is always an integer-valued from 1 to 100. The fill factor option is designed for improving index performance and data storage. By setting the fill-factor value, you specify the percentage of space on each page to be filled with data, reserving free space on each page for future table growth.

Specifying a fill-factor value of 70 would imply that 30 per cent of each page will be left empty, providing space for index expansion as data is added to the underlying table. Space is reserved between the index rows rather than at the end of the index. The fill-factor setting applies only when the index is created or rebuilt.