What does dry really mean?

[Botafogo]

"Dry" is a nuetral term meaning their is no perceptable sweetness. What you are referring to is astringency or bitterness which means that the wine has an excess (for your palate at that moment with that food) of tannin or acidity. Imagine some lightly roasted, not too extracted coffee: it is not sweet or bitter, it's just there. But, if you dark roast the beans and over extract it in a press it will be bitter and astringent but still have the same amount of sweetness (none).

The purpose of this astringency in wines (from a culinary point of view) is to balance fat in foods: a piece of meat which seems greasy by itself will both taste better with a slightly tannic red and make the red seem to be smoother to your palate. Same with acidic whites and oily or spicy fish. The best wines are one part of a binary flavor bomb and aren't MEANT to show well on their own, food is assumed (even demanded).

Enjoy your research, Roberto