Once BIOS detects the bootable device then it executes the MBR (Master Boot Recorder). MBR is 1st sector (512 Bytes) of the 1st bootable device. In 512 bytes, 3 parts are there :
i. 1st 446 bytes has primary boot loader information.
ii. Next 64 bytes for Partitions (16+16+16+16)
for this reason only we are able to make 4 Primary Partitions.
iii. Next 2 bytes for validation check of MBR. (Magic Number)
Only one of the four partitions can be marked as active. Second boot loader is then loaded from 1st sector of this active partition which gives you an option to chose your OS to boot.