A description that appeals to the senses (what is seen, heard, smelled, tasted, or touched) and makes the reader perceive the scene, not just understand it.
In Macbeth
Is this a dagger which I see before me, / The handle toward my hand? [...] I see thee still, / And on thy blade and dudgeon gouts of blood, / Which was not so before.
The passage makes the reader see what Macbeth sees: a dagger in the air, "the handle toward my hand," and then "drops of blood" on the blade. The image is visual and concrete, and it is the only one the audience receives before the crime.