It tells you when the selected file was last modified. This used to be displayed in the Properties plugin for all file types, using the same Date/Time headings, until the Properties plugin started using existing code from the Track Info screen whenever an audio file was selected. In which case it says "Track Info" instead of "Properties" in the context menu, but still launches the Properties plugin in the background.
I'm not sure Date Modified necessarily belongs in the Track Info screen. But MarcAndersen on IRC had asked for it to be brought back a few days ago, and it was easily re-added to the Properties plugin, since the plugin already retrieves this info from disk.
The Properties plugin is only used in the Database and File Browser though. WPS and playlist viewer use the Track Info code directly. Hence the discrepancy there.
Probably makes sense to get rid of this inconsistency in one way or another, and/or use strings that say "Date Modified"/"Time Modified" (or just a single "Modified" field) to make this clearer.