Directory traversal is a type of web application attack that involves exploiting a vulnerability in the web server or application to access files or directories that are outside the intended scope or root directory. It can allow an attacker to read, modify, or execute files on the target system by using special characters such as .../ or %2e%2e/ to manipulate the path or URL. In this case, the attacker used .../ to access the /etc/passwd file, which contains user account information on Linux systems.