"For what shall it profit a man if he gains the whole world but loses his own soul?"

It means that, some people do everything in order to gain power, money, fame, and luxuries in life while they are on Earth. They do not care how they got all these. Even if they were doing or gaining it in a negative way. In the Final Judgement, they cannot bring all these. their fame, money, luxuries, all and all. They will lose everything including their soul which they forgot to pray while on earth.

Nothing in this world is worth losing yourself over. There is no use in gaining something if you lose yourself, your virtues, your beliefs along the way.