"We’re building the plane while flying it"

Many of us have heard this metaphor in many a meeting. But what is the speaker actually saying about the initiative being described? Is it flying at so quick a speed that we can’t be expected to understand or question its flaws? Is this turn of phrase an excuse for haphazard execution?

Perhaps we can be more specific by identifying the pieces of the project that we’ve figured out, what we’re still working on, what we don’t know yet, and how we plan to make adjustments based on what we learn.