It gives access to fundamental resources needed for app. The Core Services layer consist of core services like Address book, Security, Social and foundation which provide essential features to apps.
These services are based on facilities in the Kernel and Device Drivers layer. The Core OS layer consist of technologies and frameworks which provide low-level services related to low-level hardware and networks. The kernel environment is built on top of Mach 3.0 (a microkernel which replaces the kernel in the BSD version of Unix) and provides high-performance networking facilities and support for multiple, integrated file systems. This is the lowest layer of iOS which mainly includes the kernel and device drivers.