Internet traffic is divided into packets. Each packet gets an address sticker that tells the network where to deliver the packet to. That address is the IP address. However, users don't want to remember IP addresses, but names. For example domain names. So they type a domain name into their browser. The purpose of DNS is to translate these domain names into IP addresses, so that the packets can be sent over the network through the server and from the server back to the browser.
In your Plesk DNS server you can create A-records, that translate a name into an IP address. Thus you tell other systems, where to send the packets to when someone enters a domain name instead of an IP address. You can also create CNAMES, that's a kind of alias (pseudonym) for a name. And you can create MX records, by which you tell other systems where to send mails to. There are several other types of nameserver records, like TXT, SRV, AAAA (for IPv6) to do a lot more magic.