Once you've created your initial design you're going to want to see what it looks like in 3D using the standard Minecraft block textures. This is what the '3D Projection' tab offers you.
The main 'Projection' section of the tab shows your current design in 3D. Scrolling with the mouse wheel zooms you in and out of the model. Pressing the arrow keys rotates you left and right around the model, and translates you up and down over the model. Holding shift down and pressing the arrow keys moves the model forward, backward, left or right in the view.
To the left of the 'Projection' view is the 'Texture' panel. This allows you to, with the simpler models, select the type of block that the model is made from. Selecting from one of the many block types in the texture panel will immediately change the 3D projection to match.