What is a TPM? (trusted platform module)
A TPM (trusted platform module) is a tamper-resistant chip, or firmware equivalent, that holds cryptographic keys and records measurements of the boot process. BitLocker uses it to release the disk key only when the machine boots unmodified, which is how a stolen laptop's disk stays unreadable while the same machine unlocks without a password prompt at the desk.
Where you meet it
Windows 11 requires TPM 2.0, BitLocker prefers it, and Windows Hello
and passkeys store credentials in it. Most machines from 2016 onward
have firmware TPM (Intel PTT or AMD fTPM) even when no discrete chip is
on the board; it often just needs enabling in UEFI. Check from Windows
with Get-Tpm or tpm.msc.
The gotcha
Clearing the TPM, replacing a motherboard, or some firmware updates change the measurements and BitLocker asks for the recovery key. That is the design working; it is only a crisis when the key was never escrowed anywhere.
Related terms: BitLocker.