A web server user is the operating-system account under which a web server or application process runs. It is often a dedicated, non-administrator account, such as www-data or apache, with permissions needed to serve content and run the application.
This account matters because a vulnerability that enables command execution through a web application may let an attacker act with its privileges. The resulting access depends on what the account can read, modify, or launch. In practice, administrators should restrict its file and service permissions, keep secrets outside its reach where possible, and monitor for unexpected processes or changes. Least privilege can limit damage, but it does not make command-injection flaws safe.