The IItemStackSupplier function can be added to a Creative Tab and is used to provide the IItemStack to be used as the tab's icon.

It might be required for you to import the package if you encounter any issues, so better be safe than sorry and add the import.
import mods.contenttweaker.IItemStackSupplier;

The IItemStackSupplier Function is a function with no parameters. The function needs to return an IItemStack.