The left column will be the place holder for many of the quick navigation tools for your visitors.
Product Search - The product search feature allows your visitors to type in the product they are looking for on your site. If they leave it blank and click go then they will be able to see all your products. You can disable and enable this feature by clicking either yes or no.
Left Navigation Images - The other portion of your left nav will be mostly for images you can insert here. Perhaps you have a sponsor or a distributor you would like to feature. You can place their logo in the left navigation (column) by browsing through your computer for the image then uploading. The other option you have is to link that image to a website. You can link the image or enter plain text into the text field and that will become the link.
The last part of your left column is your Site Map which has a few options as well.