For what? For how to use the imfill() function in the Image Processing Toolbox? Or for the source code that it uses?
To use it, you simply do
binaryImage = imfill(binaryImage, 'holes');
If you want to know at a low level what it does, you usually can't get that info since most toolbox m-files are just wrappers that pretty quickly jump into a DLL for which they don't supply source code. You can get similar code by searching on the internet.