Concrete diction is using imagery to make a better sentence.

Concrete: words which appeal to one’s senses Diction: the way one speaks

Regular Diction: It was a rainy day and Lenny jumped in the puddles.

Concrete Diction: The constant stream of water pored to the ground in sheets as little soaked through Lenny, in his favorite yellow and black rain jacket, splashed into the largest puddle he could find.