Here is the Wikipedia article on the EFI system partition.
It says, in part,
The EFI System partition is a partition on a data storage device that is used by machines that adhere to the Extensible Firmware Interface. It contains the boot loader programs for all operating systems installed (in other partitions) on the device, device driver files (used by the firmware at boot time) for other devices, and system utility programs that are intended to be run before an operating system is booted.
On Apple–Intel architecture Macintosh computers, the EFI partition is initially blank and not used for booting. However, the EFI partition is used as a staging area for firmware updates;