Robohelp creates all sorts of files in the output that aren't in the project, for example, whdata, whgdata, etc. These are all used to make the webhelp work in various situations so anything RH creates should probably be left alone. RH will recreate all the files it needs each time you generate anyway.
Someone else might be able to tell you exactly what it's for, but it contains references to various buttons, browse sequences, comments and bunch of other stuff, so I personally wouldn't remove it, just in case.
Thank you so much, Peter - that's exactly the answer I needed. We will remove it now without concern!