A bootloader is a type of program that loads and starts the boot time tasks and processes of an operating system or the computer system.
Bootloader is a preinstalled program on your Trezor device which basically verifies the validity of the firmware software being installed during its first set-up or during updating your device.
See also all about our bootloader versions and changes which were applied on this GitHub page
Trezor T initialization is split into two stages. First stage boardloader is stored in write-protected area, which means it is non-upgradable. Only second stage bootloader update is allowed.
First Stage - Boardloader
Second Stage - Bootloader
Second stage checks the integrity and signatures of the firmware and runs it if everything is ok. If second stage bootloader detects a pressed finger on the display(entering bootloader mode) or there is no firmware loaded in the device, it will start in a firmware update mode, allowing a firmware update via USB.
See also GitHub page