Validation is the most-recommended communication skill and the most misunderstood. It doesn't mean agreeing. It doesn't mean saying the other person is right. It means communicating: "what you feel makes sense, given where you're standing."
The practical formula: name the emotion, then the context that explains it. "You're upset because you worked on this for three weeks and nobody said anything." That's all. No "but" — because anything following "but" cancels what came before.
Its opposite, invalidation, is often well-meaning: "it's not that bad", "at least you have a job", "think positive". All carry the same underlying message: your reaction is wrong. The result is that the person doesn't feel soothed, they feel alone.
In DBT, validation is taught explicitly as a skill, precisely because it isn't intuitive.