What does 'counter a spell' specifically mean, and can Not So Fast counter the effects of Falling Star after it has been activated?

Ruling: Not So Fast can counter abilities, including the reflexive triggered abilities created by Falling Star. It cannot counter Falling Star itself because that spell does not target.

Sequence:

• Falling Star resolves and creates 2 reflexive triggers
• Those reflexive triggers are triggered abilities that go on the chain
• Not So Fast can counter those triggered abilities (since it counters abilities)

Nuances:

• Reflexive triggers are triggered abilities, not just effects
• Abilities have effects, but abilities and effects are not the same thing
• A spell's effect can be to create triggered abilities