Can anyone please explain more about "open resolvers"
It is nothing complicated. Just a DNS resolution service that also accepts requests from external sources, rather then just identified clients. What it does is listens for someone (anyone) to send a request to it (on port 53 usually) which might ask it to do something useful (like resolve a domain name and return a corresponding IP address) or something malicious (like return a huge number of DNS records to a target that you want to DoS). Because UDP (which is one-way in a sense) is usually used to send the request packets to the resolver and because the resolver allows anyone without identification to submit recursive queries, you can fake the sender IP in the header of the packet and the DNS resolver will send the stuff you or anyone else requested to the IP you specify in the header of the packet. So basically you can use the OpenDNS service to help you DDoS your target.
Of course openDNS service providers can do quite a lot to protect against such attacks, but many don't bother to.
Hope this helps. If you want to know more about how the technology actually works, just get some book on DNS and it should have everything you wanna know.
