Sorry about jargon.
When it is overcast with few shadows or direct light, there is very little contrast among different areas of the scene and so the response of most of the surfaces is in the same narrow range of ucky grey. Oftentimes, unfortunately there will be some bright spots like teeth and eyes and eye that determine the upper exposure on the exposure meter and the middle tones are underexposed and poorly separated.
So the picture ends up with the sky, teeth and eyes being the brightest part and everything else being mushed in the middle instead of having a lovely staircase of tones from brightest white to darkest black.
Hope this muddle makes some sense.
I tend to overexpose low contrast scenes by a third of a stop so I can actually get some saturtion in the colors.