What is Dynamic Discovery?
eduroam has traditionally used a hierarchy of RADIUS servers. All national roaming authentication traffic was aggregated into a national proxy server; all international roaming traffic was aggregated into a set of international proxy servers.
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For realms in generic top-level domains like .net, .org, .com etc. it is also RECOMMENDED to add these entries; and it may become mandatory at a later point in time.
Adding Dynamic Discovery hints to the IdP's DNS zone
To add Dynamic Discovery hints to your zone, you must first contact your national eduroam operator to determine which target name they have set up on the national proxy server; because you need to enter that discovery target in your DNS entry. In this documentation example, let's assume your national operator told you that the target name in your federation "Antarctica" is "_radsec._tcp.eduroam.aq". Let's further assume that your realm for eduroam and DNS domain is "greatidp.aq".
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Don't worry, RADIUS software knows how to interpret this further 
If you are curious though, the next section explains what all these entries mean.
NAPTR explained
NAPTR records are more complex than, say, "A" records - the A record has only one piece of information to convey, namely the IP address which belongs to a name.
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