Evan Abrams has a look into the basics for working with Value at Time expression in After Effects.

With After Effects a lot of cool things can happen with expressions. For example, Value At Time is a great way to set an animation delay for some cool effects. Watch this short tutorial from Evan Abrams who walks through what is possible when you work with the Ae property, and shows how to create a simple trail effect complete with a custom delay setting.

valueAtTime() in Ae gets the value of a property as evaluated at any given time. That may not sound like much if you are not too familiar with expressions, but Evan explains it all here. “The valueAtTime() property is one of my most used expressions in Adobe After Effects,” Abrams says. “we’ll cover the basics of how it works, and how to make a simple trail with a custom delay. Do don’t delay, and follow along with this tutorial!”