Cookies are small files that your browser stores at the behest of websites. The website which created the file(s) has access to them when responding to your attempt to view a page, and usually stores information which can allow it to identify your account in order to personalize the page response.
The Buchholz Nursery site uses a single cookie to determine which account, if any, you have used to log in.
You may wish to reference Google's instructions on how to enable cookies, or Wikipedia for a more detailed description of cookies.