Final answer:
Expressions in After Effects are based on the JavaScript language, which allows dynamic control of properties for automation and complex animations.
Step-by-step explanation:
Expressions in After Effects use a scripting language that is based on JavaScript. These expressions allow for dynamic control over the properties and can automate complex animations. Users typically add expressions to properties with a simple scripting language that includes variables, functions, and objects, which are constructs borrowed from JavaScript. Efficient use of expressions can save time and create sophisticated animation sequences that would be difficult to achieve with manual keyframing.