PCR values are the measured-boot registers inside a TPM. As firmware, bootloader, kernel, and other early components run, their hashes are extended into Platform Configuration Registers, creating a state record of what the device booted. A TPM can then compare the current PCR values to an expected policy before it releases a protected key.
This matters because disk encryption and other secrets are often tied to those measurements. If an attacker changes a boot file, tampers with firmware, or inserts an untrusted component, the PCR values change and the TPM may refuse to unlock the disk. In defense, PCR-based policy helps detect boot drift and block key release after unauthorized modification. In attacks, physical access to writable startup files can sometimes let an adversary influence the boot chain before the TPM decides whether the system still matches the trusted state.