After Effects Nomenclature
In the example above, the position property is highlighted. Notice the position coordinates are red which indicates that these coordinates are in some way controlled by the expression to the right. Expressions can take complete control of a property, turn control over to an expression control (such as a slider), or partially control the property (such as the clamp expression, where you can manually enter in data, but the expression controls the range allowed by the clamp expression’s parameters). You can also combine keyframes with expressions to achieve certain property effects.