ISO 27001 is the international standard for information security management systems. Certification means an accredited auditor has verified that an organisation runs a systematic, risk-based security programme covering people, processes and technology, not just that it owns a firewall.
In workplace software procurement it works as a trust shortcut: instead of auditing a vendor’s security yourself, you rely on the certification and focus your questions on scope and recency. Check the certificate covers the actual service you are buying.