What does trip angle mean in the context of slope / level sensors / inclinometers?
From the link that you provided, it would seem that trip angle is a preset angle at which the sensor will "trip" or "switch on". The results of this can depend on purpose of the tilt meter. It can be wired to sound an alarm, turn on a light, etc.
From this spec sheet on another tile switch, the trip angle can be adjustable. The spec sheet also gives an example circuit.
Another sensor on the site that you linked gives possible uses.