root of trust
The part trusted without a check because nothing runs before it and it can't be changed: ROM plus burned eFuses.
A root of trust is the component whose correct behaviour is assumed rather than verified -- on an ESP32, the mask ROM bootloader and the eFuse-held key digests -- from which every later verification derives.
Examples
On the ESP32, the root of trust is the mask ROM plus the eFuses burned into the chip: nothing runs before the ROM and nobody can rewrite it.
If ROM could be talked into running an unsigned bootloader, every later check would be worthless.
See also