Impact is where research changes lives, knowledge, society or the economy so for example policy has impact when it is implemented. For Applied Research these will be benefits for economics, environment, policy, health and other forms of decision making and behavioural change via stakeholder engagement. In the Pure Science, Arts & Humanities more likely to focus in terms of awareness, attitudes and cultural change and will use wider public engagement than applied research.
Plan for impact with measurable goal, well defined stakeholders/publics and a plausible plan that can be pursed with all stakeholders/public. A common mistake in planning for impact is to identify indicators that reflect the success of activities designed to create impact but not whether the expected impact was achieved or not. Make sure the indicators are measurable, reliable and robust.