Humility comes from the root word humus, which means earth, soil. The dictionary defines humus as the dark organic material in soils, produced by the decomposition of vegetable or animal matter and essential to the fertility of the earth.
Naturally and organically we are creatures of the earth; Alan Watts used to say that the earth is peopling. We came from the saltwater of the oceans and from the richness of the soil, which includes everything. It is aerated and moist because it's not separate from the air and clouds.
Humility allows you to go all the way to the fertile ground, not stick up like you're some big deal because you've got something that others don't have. When you sink all the way to the ground you actually are a Big Deal--not because you are you but because you are not you. You've sunk down to the fertile ground of all being, which is not a solid ground. It is aerated with all sentient beings, so aerated that there's no solidity at all. It is completely groundless.
With humility we may even experience the groundless ground from which all of reality arises and takes a particular shape and function for a little while in the conventional world. Then it returns to the groundless ground of interdependence and interpenetration. Humility is realizing in a visceral way that we are not separate from anything else. It allows us to sink down into the rich soil, all the way to the bottom.
The more naturally humble you are the more confident you are. It's not a childish, arrogant confidence that is based on some accomplishment that you are proud of. Relaxed confidence doesn't depend on accomplishment because it is the parent of accomplishment not the child of it.
You don't have to be right all the time. If you worry about being right then you have a very fragile confidence. But with humility your confidence is steady, it can withstand turbulent times when things don't go the way you thought they would. When you fall down you just get up. Falling down is no problem. Asking for help is no problem.
Confidence without humility is heavy, so heavy that it's hard to get up when you've fallen. It tells you that failure means you can't do it. You're embarrassed and even ashamed to ask for help because that means you're not good enough. Confidence without humility is rigid. It breaks easily because it is clinging to something specific.
Zen practice is about sinking down into the humus, where humility and confidence come together naturally to create a humble feeling of being at home in your body, wherever it is, whatever is happening. In the soil, we discover a way of being that allows us to experience the world as alive and part of who we are.