WordPress multisite subsites may be mapped to an non-network top-level domain. This means a site created as subsite1.networkdomain.com, can be mapped to show as domain.com. This also works for subdirectory sites, so networkdomain.com/subsite1 can also appear at domain.com. Before setting up domain mapping, make sure your network has been correctly set up, and subsites can be created without issues.
Before WordPress 4.5, domain mapping requires a domain mapping plugin like WordPress MU Domain Mapping.
In WordPress 4.5+, domain mapping is a native feature.