"HSM Mode" is the name we use when speaking about the Coldcard signing transactions without a person physically present. It's an advanced feature which requires additional software to setup and enable.
CKBunker is a helper program that makes it easy to reach your Coldcard over the Tor Network, and remotely submit PSBT files for signing. It also handles setup and configuration of the Coldcard's HSM features.
Learn more about CKBunker and read it's docs online, but first it may be helpful to understand Coldcard's HSM features.
You do not have to use CKBunker to make use of Coldcard's HSM features, and we expect other helper-programs to evolve in time.
HSM mode can be enabled after a "policy file" has been uploaded and confirmed by the user. That policy file establishes what the Coldcard will and won't do in HSM mode, and what further authorization steps may be needed for each spending operation.
Once the Coldcard has gone into "HSM Mode", it must be powered-down before it can operate normally again. When a Coldcard has an HSM policy file already installed, you are given an opportunity to enable it each time the Coldcard is powered, just after the master PIN is entered and before the USB connection is enabled. It is also available in the main menu, as "Start HSM Mode".
When in HSM Mode, the Coldcard screen will look like this:
No transaction information is shown on the screen, although a progress bar and one-word description of current activity (ie. "Signing...") is shown along the bottom of the screen.