When you're breaking down your script, it's often helpful for your crew to have an at-a-glance summary of a scene instead of having to read the entire thing. This is especially helpful further down the line when you make your schedule.
In Celtx's Breakdown module, you can add a scene summary to each separate scene. Just click on the scene, select the Scene Summary box in the top left, and away you go!
The text entered here will also automatically appear in your schedule for that project. You can edit it there if you want to refine it, and changes will be reflected in your Breakdown module.