What does it mean one of the stars triggers another one?

1. If you refer to "stars" as "stars" belonging to the Flying Star theory, then:-

1.1. For example, a sector may have Mountain Star #5 with Water Star #9 and base star #2.

1.2. In this simple illustration:-

#5 = earth element (significance = misfortune, sickness)
#9 = fire element (simply fire element)

1.3. In this example, the water star #9 is not good as the combination of #5 with #9 means that under the Five Elements concept Fire #9 fuels or triggers a more nastier #5. Making the misfortune sickness worse-off! Than if there is no #9 fire to fuel it.