It means:
Life is a person's most precious thing. Everyone has only one life. A person's life should be spent like this: when he recalls the past, he will not regret because he wasted his time, nor will he be ashamed because he is mediocre.
On his deathbed, he could say, "My life and all my energy have been devoted to the greatest cause in the world, fighting for the liberation of mankind.
Extended data
Paul spent his whole life practicing his life principle: "Life is a person's most precious thing, and everyone's life is only once. A person's life should be spent like this: looking back, he will not regret wasting his time, nor will he be ashamed of doing nothing.
In this way, when he dies, he can say,' My life, all my energy, has been dedicated to the most magnificent cause in the world-fighting for the liberation of mankind'. This is a true portrayal of Paul's fighting life and a profound summary of his revolutionary optimism.