New Post: What are the list callbacks?

I'm trying to implement a function similar to the ListDirectory function for SftpClient, but that is capable of listing the contents of sub-directories as well. I noticed that the signature for ListDirectory is as follows:

public IEnumerable<SftpFile> ListDirectory(string path, Action<int> listCallback = null)

What exactly is the Action<int> listCallback parameter for? I did some research and came to something about delegates and callbacks being functions that are called after the main function finishes. But I'm still a bit lost as to what their use is here.