Impact is the relative impact on your business, based on factors like money, reputation or business risk.
High impact tasks have a significant positive impact on the business, such as a big payday if completed on time, or a significant negative impact, like a grievance or sanctions if a deadline has passed.
Medium impact tasks have a smaller impact on the business. For example, launching a new marketing campaign that you think will get you 10% more business over the next 6 months.
Low impact tasks do not effect the business much. For example, you might have a task to come up with a better list of drinks you want in the fridge. It's something you might need, but it isn't going to move the business one way or another.
For example, if completing this task will result in a lot of money coming to your firm, that would be high impact. If failure to complete this task on time would result in a grievance, then this would also be high impact.
You can choose low, medium or high impact.