Abstract
Verses
Concrete
Every single thing can fall under two categories, abstract or concrete concepts.
Abstract
These are concepts that you are unable to experience with touch, scent, sight, sound or taste. The most common examples are:
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Time
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Love
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Family bonds
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Hatred
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War
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Peace

Concrete
Concrete concepts are things that you can experience with the five senses. Some examples would be:
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Dogs
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Shoes
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Nicolas Cage
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Music
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Cheesecakes
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Velvet

Why does this matter?
When writing a poem, you first want to choose the subject or the "what" you will be writing about. This "what" could be an abstract concept or a concrete concept. This choice determines the way you will describe your subject. Look at the two examples below.

Example
One
Her hair sprawls when she lays,
Long interstates winding,
Turning.
Freckles dot her face
Like the windows on buildings
Her eyes are only seen at night
In the lights reflecting off of the dirty river she loves,
Samantha
May
Marketing Executive
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Example
Two
I am a sentence
Filled with oddities and curios
Words that seem to float
With no strings attached.
Reach out
Grab some,
A jumble of thought
And churn them into a poem.
Could you tell the difference? The poem on the right falls into the classic "time describing love" problem, where you've got an abstract subject, and you describe it using abstract concepts. While many might think ambiguity is the goal, the opposite is true. In the left poem, I used a concrete thing, the city where I grew up, and used a concrete concept, a woman, to describe it.
This isn't to say that you should avoid abstract concepts altogether. Instead, try one of the following:
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Write a poem with an abstract subject and use concrete concepts to describe it.
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Write a poem with a concrete subject and use abstract concepts to describe it.
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Write a poem with a concrete subject and use concrete concepts to describe it.