what you means by concrete experience?

Answer 1

Practical, physical experience.

Answer 2

In most phrases I have seen, concrete experience means one
of two things...

It means you have had experience working with sand and cement, mortar, grout, laying bricks or blocks, pouring
cement foundations or slabs, and things like that.

OR

It means you have had a solid experience, firm encounter,
rock hard resistance, or crashed into a massive obstacle.
The word concrete in that usage might refer to the qualities of
concrete as being hard, non-yielding, heavy and solid, unmoveable.