Once you have storyboards to convey the concepts and angles to your team, the next stage is to animate these to get a sense of pacing – roughly how long should each shot last? How long is the whole project likely to be? Do we need to cut or add anything to tell the story in a way that our audience understands what is going on?
Your first animatics may be just be each storyboard frame shown for a few seconds in sequence, perhaps with some limited basic movement added using Adobe After Effects or similar. You might go on to create a basic 3D world in the free Blender, or industry standards Maya or 3ds Max, which allow you to move a virtual camera and CGI puppets and move towards a more complete understanding of the scenes. Often as each scene is rendered it is cut into your basic animatic so that you have a slowly evolving and improving video that eventually is composed of your final renders.
Framestore (of Walking with Dinosaurs fame) has a page on their pre-vis work and an excellent behind the scenes for the live action production of How to Train your Dragon.