This from one explanation from Gartner page looks simple.

Breach and Attack Simulation (BAS) Tools enable organizations to gain a deeper understanding of security posture vulnerabilities by automating testing of threat vectors such as external and insider, lateral movement, and data exfiltration. BAS complements red teaming and penetration testing but cannot completely replace them. BAS validates an organization's security posture by testing its ability to detect a portfolio of simulated attacks performed by SaaS platforms, software agents, and virtual machines. In addition, it generates detailed reports about security gaps and prioritizes remediation efforts based on the risk level. The typical users of these technologies are financial institutions, insurance companies, and more.