.idlk files are the lock files that prevent more than one user from accessing a document at the same time. They are created automatically when you open a file, and should be deleted automatically when the file is closed.
If you have any doubts about your document being saved, you should export to IDML: see Remove minor corruption by exporting and also be sure that your version if ID is fully patched.
I usually do remove those. I likely became paranoid and all of a sudden did not think I ever saw the word lock in there. I work at a home office so that has never been an issue. Thanks for that tidbit.
As for "fully patched" term. I will look into that. It is not a reference I am familiar with and I will look at your link Remove minor corruption by exporting. Thank you!
fully patched means you've run the latest update for the program.