"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.