Expressions are a programming language inside After Effects, based on Java Script. They can be enabled by an Alt+ click on the stopwatch of a different property which generates the bar for writing expressions. The expression is the value used to fill in that property, like,for example, the wiggle expression.

Let's take for example an expression for the rotation property of a redrectangle: Alt + click on the stopwatch triggers the expressions bar. By writing 40/5 in the bar, the rectangle will obtain a rotation of 0x + 8.0. Using the pickwhip, you can drag and assign the value to another property, like for example the position.



The equal (=) sign enables and disables the expression.

The advantage of using expressions is that with them you only change a single value, whereas with keyframes you have to change every single keyframe, therefore it saves a lot of time and energy.