Filtration is the process of separating unwanted material from a fluid by passing it through a barrier or medium. In cyber security, the same idea is used more broadly for screening data, traffic, or files so that suspicious or disallowed content is blocked while legitimate content passes through.
It matters because many defenses depend on filtration: firewalls filter packets, email gateways filter spam and phishing, web proxies filter malicious sites, and data-loss tools filter sensitive information leaving a network. Attackers often try to defeat these controls by fragmenting payloads, encoding content, using encrypted channels, or hiding malware in files that look harmless. Strong filtration therefore needs clear rules, good detection logic, and regular tuning so it can stop threats without breaking normal business traffic.