Multisite is a WordPress feature that allows creating a Network of sub-sites within a single WordPress installation but each sub-site will be under a separate sub-domain. So if you have purchased a domain name called MyDirectory.com and you are going to have multiple sub-domains such as sub1.MyDirectory.com, sub2.MyDirectory.com and so on, then you may purchase an only single license for one primary domain. You may have multiple sub-domains under the same single license that was purchased for the primary domain. For more details go here.