Domain mapping means telling your Web server which domains you want WordPress to answer to, and which site you want shown to the visitor when they request that domain. This process is more than domain forwarding or masking because the URLs for your posts will have the full domain name in them. Instead of the child site being in secondsite.yourdomain.com format, it can be myotherdomain.com.
Domain mapping isn't possible in certain instances, however. If your WordPress install is in a subfolder and this folder is part of the URL, then any mapped domain will also contain this folder name. In that case, it would be better to move the install so that it isn't in a subfolder.
You also need to access your Web host's control panel (where you manage DNS records on your Web server) and the control panel for your domain name registrar, which is often a different company.
The network install, by default, lets you choose between a subfolder setup and a subdomain setup. This step is still required before you can specify a domain for that site. We cover how to enable the network in Chapter 2 of this minibook. Please ensure that the network is set up and functioning properly before you attempt to map domains.
You need to set up your Web server to accept any incoming requests for the domain you want to map and the location to send them to. ...