What are examples of ‘concrete details’?

Q: A beta reader for my manuscript wrote on it that I need more “concrete details.” What did she mean?

Concrete details appeal to our five senses. All of us live in a world in which we constantly see, hear, smell, taste and touch and so also should our stories’ characters. So rather than describe a city as forlorn, you might describe the sky as overcast, the boarded windows of closed stores, the stench of garbage and urine as passing an alley, the grime on buildings as one leans against a wall, and so on.

Related articles:
• Use concrete details to make writing more vivid
• How to make your writing more vivid