What are Expressions?
Expressions are tools. Just as you use the Rotation tool and the Pen tool, you can use Expressions to control animation and composition in After Effects (AE).
Whereas you use most tools by clicking on their icons (e.g., the pointer or the pen) and clicking or dragging with the mouse, you use Expressions by typing commands on the keyboard. That’s what Expressions are: typed commands. You type these commands to tell AE what to do, and—assuming AE understands your commands—it does what you want it to do.
But you don’t always have to type to add an Expression. There are some clever click-and-drag tools that will save you time. But these tools just do the typing for you. Expressions are still typed commands.