hi,
what is the meaning of reverse dns entry
and why it is needed

is it good or bad

reverse dns is when you give an IP address (207.218.223.142 for example) and it reverse-resolves to the fully qualified domain name (FQDN) (www.ev1servers.net in this example). It's the opposite of giving the FQDN and getting the IP, which is why it's reverse. It's needed for things such as nameservers and mail (some mailservers will refuse delivery if both forward and reverse lookups do not match). If you need to set up reverse DNS for an EV1 IP, you have to go to the members area to do it. EV1 does not delegate authority for reverse DNS, so no matter how hard you try in your web panel, it won't work.
Hope this helps.

"EV1 does not delegate authority for reverse DNS" what is the meaning of this
I can not understand

It means that they will not hand over the RDNS for you to manage - it's in their nameserver, period. If they DID delegate the authority, you could set up RDNS zone files on YOUR nameservers. The best you can do is manage the RDNS through the members area, which makes the change on EV1's nameserver.