Cream of tartar is tartaric acid, commonly found in grapes.
It can be used as a mild acidulant, it doesn't impart much flavor.
I use it to invert sugar. When sucrose (table sugar) is heated in the presence of acid, the sucrose molecule is cleaved into 1 molecule of glucose and 1 molecule of fructose. Fructose is sweeter than sucrose, so the resulting inverted syrup is sweeter than a simple syrup.
Honey is an invert syrup, which is why it is so intensely sweet compared to table sugar.