Concrete and abstract things are like the mind and the body. One can be seen and the other unseen. One is observable and the other unobserved. One is physical and the other metaphysical.
When we think about love, hate, happiness, education, sadness, or feelings, for example, these are abstract things. Their meanings have different connotations for each person. We cannot measure them. It is all in our heads about what they are. It is like asking someone about his or her feeling about something.
Contrary to what is abstract, concrete things are what you can see and what you can touch. You can observe these things with your five senses. A red flower will be red to everyone observing it. If it has a fragrance, that will be true also. We can say that a certain species of plant will bear red flowers, have a special shape, or size. This is measurable. Now if we say that we think that the plant is pretty, that will be abstract thinking because what is pretty for someone might not be so beautiful to another.
We know that another word for our brain thoughts is the mind. We cannot see what others are thinking. When we think about what triggers others to behave in a certain way, we think about what is going on in the mind. The person experiencing it can reveal the negative or positive thoughts that resulted in a particular behavior. However, the behavior on the other hand is observable and therefore becomes action of the body.
We then can say that when others see what is going on around them that this is concrete thought about something. On the other hand, how they feel about it is all in the mind.