The whole sentence means: using a bronze mirror as a mirror can help us tidy up our clothes; Taking history as a mirror, we can know the change of dynasties and learn lessons from it; Only by looking at people as a mirror can we understand where we are not doing well and where we are doing well.
Extended data:
The allusion of Emperor Taizong's saying this sentence:
In the seventeenth year of Zhenguan in the Tang Dynasty (AD 643), Wei Zhi, who dared to speak out, died of illness. Emperor Taizong was very sad. He said with tears: "My husband can dress himself up with copper as a mirror; Taking history as a mirror, we can know the rise and fall; Take people as a mirror, you can know the gains and losses. Wei Zhi didn't, I died a mirror! "
"A person who takes copper as a mirror can make his clothes and hats wear correctly; Taking history as a mirror, we can know the reasons for the rise and fall of the country; Looking at people as a mirror, you can find your own right and wrong. After Kevin·Z died, I lost a good mirror. " -This is the highest evaluation of Wei Zhi's life value. There is also a saying that "taking history as a mirror, we can know the rise and fall".
References:
Baidu encyclopedia-taking copper as a mirror