A communication protocol is the agreed set of rules that lets systems exchange messages in a predictable way. It defines how data is formatted, sent, received, acknowledged, and interpreted, so both sides understand what each message means.
Protocols matter in cyber security because security controls depend on them. If an attacker can spoof, alter, or confuse a protocol, they may intercept traffic, impersonate a device, trigger a downgrade to weaker encryption, or exploit parsing bugs in implementations. Defenders use secure protocol design, strict validation, authenticated encryption, and careful version handling to reduce those risks. In practice, protocols appear in everything from email and web traffic to remote access and industrial systems, so understanding them is essential for spotting abuse and building reliable defenses.