GetInfoTip is defined as follows:

function GetInfoTip(dwFlags: DWORD; var ppwszTip: PWideChar): HResult; stdcall;

.dwFlags – currently not used
.ppwszTip – Address of a Unicode string pointer that receives the tip string pointer.

Important Note
The ppwszTip parameter of the GetInfoTip method is a pointer to a Unicode string buffer that contains the text to display in the tip. This buffer must be allocated using the standard shell memory allocator because the buffer is allocated by our application, but freed by the shell. To ensure that everything happens in a thread-safe way, use SHGetMalloc to get the pointer to the shell's memory allocator – an object implementing IMalloc. Then, use IMalloc's Alloc method to allocate the memory needed to hold the Unicode representation of the Infotip text.
The accompanying source code contains the standard code you will use for all Infotip Extensions you create. Simply use the same code and your extensions will be shell friendly and thread-safe.

IPersistFile is what the shell uses to provide the extension with information about the file the user is hovering over. The interface defines five methods:

.IsDirty – checks an object for changes since it was last saved to its current file. We don't need this for Infotip extensions so we return E_NOTIMPL.
.Load – opens the specified file and initializes an object from the file contents. We use this method to retrieve the name of the file the user is hovering over.
.Save – saves the object into the specified file. We don't use it, so return E_NOTIMPL.
.SaveCompleted – notifies the object that it can revert from NoScribble mode to Normal mode. We don't use it, so return E_NOTIMPL.
.GetCurFile – gets the current name of the file associated with the object. We don't use it, so return E_NOTIMPL.

Load is defined as follows:

function Load(pszFileName: PoleStr; dwMode: LongInt ): HResult; stdcall;

.pszFileName – points to a zero-terminated string containing the absolute path of the file to open. .dwMode – specifies some combination of the values from the STGM enumeration to indicate the access mode to use when opening the file.

We are only using the IPersistFile interface to obtain the path and filename of the file; we are not actually using the interface to access the file so we can ignore the flags. Our standard implementation of the Load method will be to store the contents of pszFileName to a private variable to be used by IQueryInfo::GetInfoTip to locate the file.