Crema is the golden brown foam on top of the espresso. Cream can tell something about the espresso, if the crema is very thin or there is not any, espresso is most likely done improperly.
Crema is formed when oils and fats of coffee are mixed with water in the immerse pressure of espresso making. The color of cream should be golden brown and texture should be velvety. Color can vary depending of the coffee used in the espresso.
If crema in not formed on top of the espresso, grind size might be wrong or the coffee is too old. High pressure is needed to form crema, for example from an espresso machine. Espresso machine pushes hot water trough very finely grinded coffee.