Definition
The impact of an argument is why it matters, stated as a concrete consequence a judge can compare against the impacts on the other side of the round. A claim can be true and a warrant can be sound, but if a debater never explains what actually happens as a result, a judge has no way to weigh that argument against a competing one.
Example
Claim: the policy reduces emergency room wait times. Warrant: it moves routine cases into lower-cost clinics, freeing emergency capacity for genuine emergencies. Impact: patients having a heart attack or stroke get seen faster, which measurably improves survival rates. That last sentence is what lets a judge weigh this argument against, say, an opposing case about cost, because it states the human consequence in comparable terms.
Common mistakes
Debaters often state an impact that is technically true but too small to matter, or too vague to compare against anything. "This is bad for the economy" is not an impact a judge can weigh; "this raises unemployment in the affected sector by an estimated amount" is. Whenever possible, state impacts in terms that make comparison easy: how many people, how severe, how likely, and how soon.