Security in this context addresses multiple concerns, both physical and logical.
From a physical security perspective, cloud service providers equip their datacenter facilities with hardware such as cameras, gates, locks, and equipment cages. They'll also implement personnel and procedures, such as guards and identification verification, to ensure only people who require access are allowed in the facilities.
https://docs.microsoft.com/en-us/azure/security/fundamentals/physical-security.
Just as importantly, computing service providers secure electronic data. To ensure the utmost security, providers implement multiple layers of logical security, including secure protocols and encryption to protect data that is both at rest (sitting on physical media) and in transit (as it is being transmitted between endpoints). These security measures help prevent unauthorized access to data. In the event of breaches of physical security, cryptographic technologies can be used to prevent attackers and thieves from being able to access the contents of stolen equipment.
https://docs.microsoft.com/en-us/office365/securitycompliance/office-365-encryption-in-the-microsoft-cloud-overview.