A traffic relay is a device or node that forwards network traffic for someone else. It may simply pass data along, or it may act as an intermediary that hides the original source and destination from the rest of the network.
In cyber security, traffic relays matter because they can be used for legitimate routing, but also for abuse. Attackers use compromised devices, proxies, or relay servers to send spam, carry ad fraud, or mask command-and-control traffic so it appears to come from a normal home or business connection. Defenders look for unusual outbound connections, unexpected bandwidth use, and devices that behave like silent intermediaries. When a consumer device becomes a relay, it can create a residential proxy path that is harder to block and easier to trust than datacenter traffic.