Shoshin (初心) is a word from Zen Buddhism meaning "beginner's mind." It refers to having an attitude of openness, eagerness, and lack of preconceptions when studying a subject, even when studying at an advanced level, just as a beginner would.
We never actually stop beginning. We just forget every now and then as we get older and -- if we're not very careful -- more foolish. Perhaps we put on a black belt and start to think we are an expert. Perhaps we start to believe the hype and think we really know something about something. But really, our goal is to cast off trappings (emphasis on trap) like certificates and belts: they are helpful, but once they get in the way of our learning they are devastating. Once they give our egos a foothold, the road-map of rank is going to run us off a cliff.
When we strive to be newborn to each moment, we find the wonder in all that we do. Assumptions and knowledge of how the world "is" prevent us from seeing clearly. Expertise, while laudable, can keep us from seeing anything outside of the boundaries we have defined by our studies. How many fighters, for instance, have been unable to adapt to an unorthodox opponent?
As with enzan no metsuke, see the big picture without judgment or assumption. Keep a beginner's mind and retain the wonder of sincere practice and new learning. Be a student of everyone, and always wear a white belt in your heart and mind, if not around your waist. We can learn every moment if we are open to the opportunity.
A historical example of the embodiment of this principle is Chao-Chou. When he was 60, the great Zen master Chao-Chou left his monastery to wander the countryside. When he departed the monastery, he is quoted as having said, "Even with a seven-year-old child, if he is superior to me, I shall follow him and beg for his teaching. Even with a hundred-year-old man, if he is inferior to me, I shall follow him and teach him." This is beginner's mind, shoshin: doing that which is appropriate regardless of the ego, letting go of the self to become the self. This is our practice in the dojo, and everywhere. It's an impossible task, and it's already happening for all of us. As Shunryu Suzuki said, "Each of you is perfect the way you are...and you can use a little improvement."