My father used to share that phrase with me occasionally. Like a lot my father's wisdom, I often misunderstood it, or applied it incorrectly. But, used properly, it is wisdom.
I have come across an interesting personality type in coaching lately. This personality probably existed before, I just didn't recognize it until recently. It manifests as an extreme desire for correction. If I offer feedback to a student, they want more. They want to know what else they are doing wrong, each and every time. I've had students say to me "No, no. What else am I doing wrong?"
On the one hand, there's something admirable about this mentality. I have a perfectionist streak myself, and I can certainly understand and sympathize with others who share that desire to make sure that every movement is done as well as it can possibly be done.
On the flip side, sometimes the obsession with perfection actually gets in the way of progress. There's only so much information the brain can process at once, and only so many corrections that a person can make in a given day. Between adjusting hand position, foot position, range, stepping, hip rotation, angle of impact...the list can keep going for a long time if you let it. At some point, there are so many errors that trying to correct them all just becomes an impossibly frustrating task for both student and teacher.
Perfection isn't going to come in a day. Sometimes, the best you can do is get a little better. Fix the other errors next time.