What is Lockdown module?

The Lockdown module is dedicated to hardening the softer areas of a WordPress installation.

Can I disable the Rest API and XML-RPC Systems?

Yes, you can. Is it entirely advisable? This depends on your usage.

The vast majority of sites can disable XML-RPC. But if you use the WordPress phone apps, then you need to keep it on.

Also, with the Rest API, more and more plugins are starting to use it, so if you disable it here you may run into trouble with other plugins.