An abstraction is pretty much anything other than an object or the action of an object.
Objects and actions are very widely understood because we can make sure we all agree about what they are by pointing at them. We can say 'chair' and immediately have a rough idea what the topic is.
You might think the same would be true of dignity, love or anguish. But thousands of emo poems seem to suggest otherwise.
Chair, wooden, yellow, wicker is concrete
pain, anguish, extreme, hurt is abstract
The true joy, IMHO, is somewhere between. Even if I can picture the chair, why do I care about it? Even if I accept the writer is in abject anguish -- without knowing who they are and why they suffer, why do I care?
If the chair was the one in which the poet's dead mother once habitually sat, perhaps we are getting somewhere.
Why do people complain more about overly abstract poems than overly concrete ones? Because there are vastly more overly abstract poems out there than overly concrete ones. And, I believe, because the abstract is often rapidly implicit in even the most concrete subject--the reader brings their own abstract emotion in every reading.
e.g.
This Is Just to Say
I have eaten
the plums
that were in
the icebox
and which
you were probably
saving
for breakfast
Forgive me
they were delicious
so sweet
and so cold
The Red Wheelbarrow
so much depends
upon
a red wheel
barrow
glazed with rain
water
beside the white
chickens.
I would argue that is it a greater failure for the poet to neglect the concrete than to neglect the abstract but in a perfect world the dichotomy between the two fades away altogether:
(NB, 'seasons' are an abstraction of concrete changes in temperature and nature, the 'child' is a reification of the message to abstract 'youth' in general. The poem starts being about leaves and ends up being about mortality. This is the very poem that first made me want to write poetry.)
Spring and Fall, to a Young Child
Margaret, are you grieving
Over Goldengrove unleaving?
Leaves, like the things of man, you
With your fresh thoughts care for, can you?
Ah! as the heart grows older
It will come to such sights colder
By and by, nor spare a sigh
Though worlds of wanwood leafmeal lie;
And yet you will weep and know why.
Now no matter, child, the name:
Sorrow's springs are the same.
Nor mouth had, no nor mind, expressed
What heart heard of, ghost guessed:
It is the blight man was born for,
It is Margaret you mourn for.
p.s. examples from William Carlos Williams and Gerard Manley Hopkins
p.s.s. but what would I know?