Boardloader is a preinstalled program in Trezor Model T stored in write-protected area, which means it is non-upgradable. Its main function is to check validity of bootloader which is upgradable.
Trezor T initialization is split into two stages.
First stage - Boardloader
Boardloader checks the integrity and signatures of the second stage and runs it if everything is ok. If first stage boardloader finds a valid second stage bootloader image on the MicroSD card (in raw format, no filesystem), it will replace the internal second stage, allowing a second stage update via SD card.
Second stage - Bootloader
See also GitHub page