On MediaTek (MTK) devices like the Redmi 6 (MT6762/Helio P22), the is the first-stage bootloader. It is stored in the EMMC’s boot partition. Its job is to initialize DRAM, load the secondary bootloader (LK), and manage the USB download protocol (BROM/DAA) for flashing.
Learn more about unbricking procedures using MTK tools from this guide on redmi 6 preloader file
: The standard utility for flashing firmware to MediaTek devices. MediaTek (MTK) Bypass Tools On MediaTek (MTK) devices like the Redmi 6