Double click on the filename or on a model in the preview area to view and edit its properties (rotate, scale, position, dimensions). The checkboxes next to the file names hide and show that file in the preview area. Use Import or drag and drop files into the list area to add files. The Models area is where you specify the 3D model files you want to work with. There are a number of tutorials on Simplif圓D's web site that cover various aspects of the software. I am using a Mac some of the keyboard shortcuts may be different on Windows or Linux. It is not officially sanctioned by or affiliated with Simplif圓D. This document is an attempt to fill some of the gaps.
It has a number of useful features, but figuring out what everything does can be a daunting task for someone who is not already familiar with 3D printing. Simplif圓D is a commercial slicer and 3D printer host.