What is a "progressive elaboration"

Definition by PMP:
"Continuously improving and detailing a plan as more detailed and
specific information and more accurate estimates become available as the
project progresses, and thereby producing more accurate and complete
plans that result from the successive iterations of the planning process."

Projects may be temporary endeavors but they cannot be conceived in a
day and for sure cannot be executed/completed in a day (unless you
define going to the grocery store as a project)
Usually there is a concept first and then a broad vision for the end
product, i.e., the outcome or result of the project.

The clearer vision you have of the unique product that you want from the
project, the more accurate the project plan will be. So, you move toward
the final project plan in incremental steps as the ideas about the final
product are refined and as you get more and more information about the
requirements in a progressive fashion. This procedure of defining (or
planning) a project is called progressive elaboration.