DNSSEC ("Domain Name System Security Extensions") is a protocol standardized by the IETF to solve certain security problems related to the DNS protocol.
When the browser sends a request, it returns with an authentication key that certifies that the returned IP is the correct one.
The user is then assured of accessing the right site when he receives a DNSSEC-validated IP. The goal is to point out inconsistencies in requests.
If a hacker tries to modify the table contained in the DNS server protected by DNSSEC, the latter refuses his requests, the information sent being unsigned.