EnterraHost
Nameservers are the addresses of the servers that hold your DNS. Changing them moves the authority for a domain from one set of servers to another, and that is a different job from editing the records inside them.
The toolbox shows the nameservers in use on the Domains page and lets you change them from there. The column is labeled DNS, and it names the provider currently answering for the domain.
Before you change anything, know which job you are doing
Two things look similar and are not.
Changing nameservers moves the domain to a different DNS provider. The records do not come with it, so whatever answers for the domain afterwards has to be given the records, and until it is, the domain resolves to nothing.
Changing records edits the DNS where it already lives, leaving the nameservers alone. That is managing DNS records, and it is the more common task by a wide margin.
If you are moving to a third-party DNS provider such as Cloudflare, both jobs happen, and in a particular order. Create the records at the new provider first, then move the nameservers. Doing it the other way round leaves the domain resolving to nothing in between.
What the toolbox changes for you
The nameserver fields are pushed to the registry on your behalf. You do not need an account with the registry, and you do not need to know which registrar holds the domain, because the toolbox already does.
There is one thing worth knowing about where the change is applied. Some registry operations are performed through the registrar's own control panel rather than directly, and a small number of extensions restrict what can be changed at all. Where a control is unavailable for a specific domain, the toolbox disables it and says so rather than failing when you press it.
How long it takes
Longer than people expect, and the delay is not ours.
Resolvers across the internet cache the answer for your domain, and they keep the old one until its time to live expires. A change made now is visible immediately to a resolver that has no cached answer, and invisible to one that does. For a period, both the old and the new nameservers are live as far as the wider internet is concerned.
That is why a domain can appear to work for you and not for somebody else, or the other way round. It is normal, and the fix is to wait rather than to change it again.
A DNS lookup from five regions is the honest way to watch it, because one lookup from one place only tells you what that one resolver believes.
Where glue records fit
Most domains never need them. A glue record is required when the nameserver for a domain is itself at that domain, because otherwise the registry has no way to resolve the name without asking the very server it is trying to find.
If you are running your own nameservers rather than using ours or a third party's, this is the case you are in, and the toolbox exposes the glue fields alongside the nameserver ones.
Changing them back
Nothing here is permanent, and the previous values are recoverable from the change history on the domain. If a move leaves the domain unresolved, putting the old nameservers back restores it — but the records the old provider was serving have to still be there, which is the reason not to cancel an old DNS service on the same day you move away from it.
Leave the old provider in place for a few days after the change. It costs very little and it is the only way back that does not involve rebuilding records from memory.