There is nothing you really NEED to know since they are the professionals, but the more information you learn in the beginning will make you that much more helpful to the design team or freelance designer you hire for the project and may save you money down the road. Having previous web design knowledge will also allow you to keep up with progress and development steps as the project gets underway, and doesn’t leave you at the mercy of your designer’s time. In addition, you will be able to free up some of your designers time by making smaller changes yourself and you will gain a better understanding of the inner workings and capabilities of your eCommerce platform and storefront. Besides, you can really never know too much about one thing.
Read some books, blogs and magazines surrounding the topic and become familiar with the lingo designers use. If you decide to pursue knowledge about web design further you can start applying some of the things discussed in the books, blogs and magazines you read. The best designers got to be that way by practicing and applying the languages they learn in real life applications and web sites. Play around, have fun and keep coding as much as you have time for.