It wants you to put in whatever nameservers you will be using. It's pretty nice, it's even offering you glue records if you're to self host your DNS too!
Most domain registrars tend to also offer DNS services and even default to using theirs, so it's often thought those come together. It seems like eu.org doesn't. So you have to provide your own. That could be Cloudflare or any number of DNS providers out there.
Most of those DNS providers will give you two name servers that you can input there. Minimum is 2 but some have 4 and 8 too, but it's rare. You just put them there for the first two and you can leave everything else blank.