A records, MX records etc are all part of the DNS records. Hackers may try to spoof these records and make any DNS query redirect to the wrong domain. DNSSEC is a security feature where the DNS records like A record, etc are encrypted. When a DNS query is made, the DNS information is revealed through decryption via DNSSEC keys.
This process is very tedious if done manually with each record in a DNS zone file needing to be signed. Our DNSSEC extension for Plesk streamlines this process making it easy to manage.