I see that term sometimes when people are talking about Wintersun, but to me it honestly makes no sense. I mean come on; over produced? Just... can someone please tell me what that means and how it makes sense.

Well, considering the fact that they needed their own private studio and fifty thousand in crowd funding to make one album... and it's not Time II...
I think what people generally mean by overproduced is it sounds too perfect and not gritty or raw enough, so that's definitely a matter of personal preference.

5849482
I'd rather have an over produced album than an under produced one.

As for the whole crowd funding/private studio thing, I feel as though that's just Jari being a cunt.

5849505 it's more of an elitist school of thought because metal is supposed to get away from genres that use "fake" techniques to make their music sound better (for example, having the same singer sing multiple things at the same time)
Raw music has its place but it shouldn't really make a difference.

5849505
5850039

Sometimes over-production is what makes an album sound like crap. They add fifty guitars on either side, 30 harmonies and all sorts of exotic instruments into every song (something they could never replicate live), the singer autotunes their voice into places it shouldn't go, and everything is made to sound so crystal clean that it just becomes awkward to listen to. With certain genres, the "poor quality" is actually part of the sound. There's a difference between being able to get the cheap equipment to enhance the experience, and recording decent songs with shitty equipment.

To give examples: a "garage" sound would have been pretty horrible for Pink Floyd's albums (beyond the first one, at least). At the same time, compare 80s Metallica to today. Song quality aside, everything just sounds sterilized, not to mention the stupid loudness war makes it impossible to find any subtle textures in the songs (this is a bit difficult to explain, but just try and listen to an 80s and a 2010s song side by side).