I found a product manual [1] from one manufacturer which describes stepping on it as part of a maintenance test (bottom of page 6), so it seems like some type of switch or pressure.

The idea seems to be that if the door touches the top of the sensor, it automatically lowers the jetbridge a few inches to prevent the door from being damaged. And I guess this can happen if the plane is being loaded improperly causing it to shift.

[1] PDF, 4MB: https://www.acesincusa.com/wp-content/uploads/2020/07/Safety...