Hi Terri,
I'm Kris, a Shopify guru.
You are correct in that cart reservations last 5 minutes, so if a customer adds a product to their cart it will be saved for them during this time to complete the purchase and eliminate the chance of overselling, according to your inventory levels.
Products that are saved in the cart are kept there by browser cookies, so they will remain until the cookie is cleared by the shopper's browser. For this reason, the time frame will vary, but the cookies are set to automatically clear when someone reaches the "thank you" screen.
This makes it as easy as possible for your customers to come back and purchase items left in the cart, assuming they are still in stock. If a product is no longer available, an out of stock warning is generated before the checkout can be completed.
Hope this helps "clear" things up for you! Feel free to contact us if you have any other questions. :)
Kris