Fill factor is the value that determines the percentage of space on each leaf-level page to be filled with data.
In an SQL Server, the smallest unit is a page, which is made of Page with size 8K. Every page can store one or more rows based on the size of the row.
The default value of the Fill Factor is 100, which is same as value 0. The default Fill Factor (100 or 0) will allow the SQL Server to fill the leaf-level pages of an index with the maximum numbers of the rows it can fit.
There will be no or very little empty space left in the page, when the fill factor is 100.