The impact lets you know how the overall avarage rating of your product was affected by the specific review. Larger negative impacts require your immediate attention.
Before the new review came in:
Four existing 5-star reviews => 4x5 : 4 = 5 => the rating is at 5
After the new 4 star review comes in.:
The calculation becomes (4x5 + 1x4) : 5 = 4,8 => the rating is now at 4,8
The impact of that review is the difference between 5 and 4,8 => -0,2