Wines described as jammy tend to have flavors and aromas of cooked or stewed berries, plums, or cherries.

It can be a more negative descriptor as it often suggests that the grapes were harvested overripe, and that they lack freshness. This can happen in warm climate regions, with excessively warm fermentation temperatures, or as a result of carelessness in the storage of wines.

However, some people love these ripe, rounded fruity notes, and when done right – it can be delicious. A jammy descriptor is often used for wines like Merlot from California’s Central Valley, or Primitivo from souther Italy’s Puglia region.

Source.