Constructive feedback is letting people know in a helpful way how they are doing, and how their performance is being perceived. Constructive feedback can be positive (letting someone know they're doing well), negative (letting people know about ways in which they could do better), or neutral (just an objective observation or analysis).
There are two main elements that make feedback (particularly negative feedback) constructive.
1. The content of the feedback: Constructive feedback is specific, behavior or issue-focused (rather than a value judgment about the individual), based on what is observable (rather than assuming anything about the person's attitude or motivation), and includes some specific direction on how to make improvements if some are needed.
2. Most important, how the feedback is delivered. To be constructive, feedback should not be delivered in a manner that provokes resentment, resistance, defensiveness, hurt feelings, shame or a sense of failure. It means not backing the person into a corner with attacks. Honest doesn't mean tactless. This is where emotional intelligence really makes a difference.