Literally "grind an iron bar into a needle".
When Li Bai, the famous Chinese poet, was a child, he didn’t work hard, and even wanted to quit studying. He met an old woman on the road one day; she was grounding an iron bar and said she would grind it into a needle. Li Bai was deeply touched, came back to study hard, and finally achieved great success. As long as there is determination, willingness to make an effort, we will overcome the difficulties and make progress.
English equivalents are： 'rub a pestle into a needle - perseverance will prevail'.
'With time and patience the leaf of the mulberry becomes satin.'
'Constant dropping wears away a stone.'
Example sentence: 学习汉语要有磨杵成针的精神。只要坚持，就一定能学好。 Xuéxí hànyǔ yào yǒu mó chǔ chéng zhēn de jīngshén. Zhǐ yáo jiānchí, jiù yídìng néng xuéhǎo.
'Keep the spirit of grounding an iron bar into a needle and persist in studying, and surely you will make achievements in learning Mandarin.'