Definition - What does Bitter mean?
Bitter is one of the most basic wine tasting notes and is most commonly caused by the tannins present in the wine. The bitter taste should not be present in older, mature wines and should never dominate the finish or flavors. The more tannins present the bitterness will increase in the wine and young red wines are more commonly going to exhibit a bitter taste.
WineFrog explains Bitter
Some grapes naturally have a more bitter taste like the Muscat or Gewurztraminer varietals but most of the time the stems, seeds and skins create the bitterness. For sweet wines, a slight bitterness may bring out the flavor but should not overpower it. Too many tannins can be due to over processing, long fermentation or blending the must instead of only crushing the grapes. An example of a wine that exudes a bitter taste is the Amarone wine from Italy, also Cabernet Sauvignon has the highest amount of tannins so it has one of the most powerful bitter tastes. Tannins act like a natural preservative which is why older wines do not taste as bitter and red wines that have higher tannins can age longer.