Known view displays all characters and words you have marked as Known. Keep in mind that 'Known' in Chinese Toolbox means characters and words that you are at least beginning to know; you can usually recognize them and you want to practice reading them in context without dictionary assistance. Reading these characters in stories or articles will help to keep them fresh. If you see these regularly, you're not as likely to forget them.
Right clicking on the Known tab displays a context menu where you can specify sorting. If you keep the Known characters sorted by date, those that you haven't read recently will appear at the beginning of this list, further helping to keep all Known characters fresh in your memory.