With domain forwarding, you can redirect a domain or subdomain to another address. This means that when one of your visitors goes to your domain/subdomain, they will be automatically sent to the destination you have set. Please note that we also use Redirect as an alternative term for domain forwarding.
You registered the domain example.com at 1&1 IONOS, but you want your visitors to see the contents of example.co.uk instead. In this case, you would set up domain forwarding.