During this step of the Live File Compression wizard the program compresses your files. The program uses the internal compression engine of Windows to compress the files.
The program shows you the size of the file, and also the size after compression, and also how much space you saved.
Even after finishing this step, Windows will continue to process your files in order to compress them. This is because Windows doesn't compress the files immediately. After this step is finished, you can work on your computer normally and/or shutdown your computer at any time. Just keep in mind that after the compression phase finishes, you may still see disk activity.
More information about Live File Compression