A term from Zen. The idea is that a beginner's mind is open. From Zen Mind, Beginners' Mind by Shunryn Suzuki 'In the beginner's mind there are many possibilities, in the expert's there are few'.
I think teachers should always try to keep the beginners mind, so we rediscover our subject over and over again, and don't ever forget what it was like when we were just starting out.