The M108 command requires EMERGENCY_PARSER for full effectiveness. (Otherwise a full queue blocks the parser.)
Some G-code commands cause Marlin to go into a closed loop, waiting indefinitely for a certain state or event. For example, M109 waits for the target temperature to be reached, and M0 waits for an LCD click.
In the case of M109, the M108 command stops waiting for the target temperature and continues processing G-code. This may result in "cold extrude" messages. For a full stop use M112.
In the case of M0 the M108 command acts like the LCD button, breaking out of M0 and continuing to process the G-code queue.
With both EMERGENCY_PARSER and HOST_KEEPALIVE_FEATURE enabled, hosts will be able to prompt for continuation or cancellation, confirming with M108 and cancelling with M112.
Use M108 as a "Continue" button in your host software.