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.