Triggers are events that occur on a specific action/circumstance.
In the case of the platform behavior the animation triggers are actually action that can be used to determine a different "state" of the platform behavior, at the moment it happens.
"On fall" for example does trigger the moment the object the behavior is applied to does start going down, without a platform beneath its feet, falling.
"Is falling" can be tested and be true every tick of execution while the object is falling.
You can find more about general conditions in the manual article on conditions : scirra.com/manual/76/conditions