In debate, the impact of an argument is why it matters - what will happen in the real world if the argument is true, and how big the consequence is. Impacts are what judges weigh against each other when deciding who won. An argument without an impact is technically correct but strategically useless.
Two Dimensions of Impact
Judges weigh impacts along two axes: magnitude and probability. Magnitude is how big the effect is - how many people are affected, how severe the harm is, how permanent the consequence is. Probability is how likely it is that the impact actually happens given the argument. A small impact with high probability can outweigh a huge impact with low probability.
Terminal Impacts
A terminal impact is the final consequence you are defending - the thing that matters in itself, not as a means to something else. Reducing poverty is a terminal impact. 'Improving one specific regulation' is not a terminal impact unless you connect it to a bigger harm or benefit. Good debaters always trace arguments out to their terminal impacts.