Greg Larsen explains how instant file initialization works:
Instant file initialization is a SQL Server setup option that keeps SQL Server from writing binary zeroes to data files when a database is first created, expanded, or restored. By avoiding the writing of binary zeroes, there is a lower performance impact when disk space is allocated for several database operations.
By default, when SQL Server creates a database, increases the size of a database, or restores a database, it needs to initialize the disk space prior to allocating the space. This initialization process writes binary zeroes (“0”) across all the bits and bytes of space being allocated. By writing binary zeroes across the disk space, the SQL Server engine makes sure that data previously stored in the unused disk space is completely overwritten before the disk space is allocated to a database.
Read on for more details, as well as why this can be worth enabling.