How can a program open a folder?

Question

I was trying to digest the info at Windows memory management revealed part one, part two but anyway on to the question. The website introduces a program called Process Explorer, and in the download page it says:

Ever wondered which program has a particular file or directory open?

Now I was thinking what does the above sentence mean.

I know what it means to have a file open, but

What does it mean for a program to have a directory open?

Answer 1

In the context of what ProcExp will show you, a program having a folder or directory open means that the program is using files inside that folder or directory or is holding the directory in memory so that it can use files that may end up in that folder or directory.

In 99% of the cases, this means a shared resource, a library or a dll, exists inside some directory such as %programfiles%\Common Files\ or the %windir%\system32 directory and that programs are using those files in order to accomplish their purposes.