Mustard here means useful people, mustard, refers to the hay on the roadside, a section of hay. Metaphor is a worthless thing, which is not cherished enough.
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The ancients often used mountains as a metaphor for a spirit. The true meaning of "mountain" refers not only to lofty mountains, but also to lofty virtue. The real moral of the mountain is not to put yourself in a high position and look down on others, but to really improve yourself from the inner world, and constantly let yourself go further, not limited to the present life, and take the first step, and our future life will be better.
If you really want to accomplish something, if you can, start now, not tomorrow, not next month, now and then stick to it. Even between today and tomorrow, there are many accidents, so don't wait.