It's exactly as the name implies: these are first entries in series that are extremely different to almost every game that would come later in said series. This can be down to a multitude of reasons really: they feature entirely different gameplay than later entries, are totally eclipsed by their sequel(s) in terms of quality or just have a wildly different atmosphere/tone than what would come later down the line.

Always open for suggestions on lists like these as I've probably missed some so if you have any ideas on what to put on here, let me know :D