Penetration testing (or pen testing) is a security exercise where a cyber-security expert attempts to find and exploit vulnerabilities in a computer system. The purpose of this simulated attack is to identify any weak spots in a system’s defenses which attackers could take advantage of.

This is like a bank hiring someone to dress as a burglar and try to break into their building and gain access to the vault. If the ‘burglar’ succeeds and gets into the bank or the vault, the bank will gain valuable information on how they need to tighten their security measures.

Cloudflare glossary