"Concrete" things are solid. A "concrete plan" is realistic and detailed. It includes specific ideas about:
- what to do
- how to do it
- what order to do each step in
Here are some examples of how to use the phrase "concrete plan":
We need to put together a concrete plan for how we're going to deal with this.
A: What are you doing for the holidays this year?
B: I don't have any concrete plans.
The opposite of a "concrete" plan is a "vague" plan.