A UDP flood is a Layer 4 (transport) attack vector, one of the volumetric classes a thorough DDoS test is built to exercise. The attacker sends a high rate of UDP datagrams to random or fixed ports on the target. Because UDP is connectionless, the host receives each packet, looks for a listening service, finds none, and answers with an ICMP destination-unreachable message. That lookup-and-reply cycle burns CPU and link capacity, and the return ICMP traffic compounds the load until legitimate packets are dropped.
Why it matters in DDoS testing
A UDP flood characterizes two limits at once: the bits-per-second a link can absorb and the packets-per-second the kernel and NIC can classify before they saturate. Testing measures the point where the interface queue overflows and where stateless filtering (dropping UDP to closed ports at the edge) actually holds. The volumetric mechanics are detailed in Understanding DDoS Attack Vectors.