The Tzimmes (tsimmes) in question is a slow cooked root vegetable stew (mostly carrot, but there are also versions that include sweet potato or pumpkin, and originally also turnip). The vegetables are cooked in fat (originally goose fat, nowadays usually oil or butter) and the sweeter ingredients are added: cinnamon, prunes, orange juice or honey, according to the recipe and particular custom. There are meat recipes and vegetarian ones. Among Ashkenazi Jews it is customary to eat Tzimmes on festive occasions, especially Rosh Hashanah and Shabbat. The carrot rounds glistening with oil symbolize desired abundance.
And why “make tzimmes out of it”? It comes from the idea that even something simple as simple as a carrot can be made into a mess (tzimmes). It can also be use in the phrase “I will make tzimmes out of him”, similar to the English “I will make mincemeat out of him”, implying the complete elimination of one as a threat.