Complex sentence: HOIC primarily performs a denial-of-service (DoS) attack (or when used by multiple individuals, a DDoS attack) on a target site by sending excessive traffic in an attempt to overload the site and bring it down.
Split into simpler sentences: HOIC primarily performs a denial-of-service (DoS) attack and a DDoS attack when co-ordinated by multiple individuals. The denial-of-service (DoS) attack on the target URL is accomplished by sending excessive traffic in an attempt to overload the site and bring it down.