IPv4 is the old standard of Internet Protocol addresses that you're probably familiar with (something like 192.168.1.10, although the 192.168.x.x range is actually only private addresses). It's still used on most devices today but it only supports up to ~4.3 billion addresses and as you can imagine, those addresses have basically run out with all of the various devices and servers and whatever else is connected to the internet. IPv4 is in the process of being replaced by the new (~~10~~ 27* year old) IPv6, but there are still a lot of old devices and a need to support the old protocol. Making IPv6 available for this server will mean any devices connected to the internet should be able to communicate with the server on this new protocol.

As for DNSSEC, when you go to resolve a a hostname, like lemmy.world, your computer will make a request to a DNS server to figure out what IP address it needs to navigate to in order to access the server. In theory, someone could intercept your DNS request and tell you an address for the server that isn't actually the address for the server, but rather a malicious host. DNSSEC basically acts as a layer of security to help confirm that information you're getting on your DNS request is good and true.