Before success comes in any man's life, he's sure to meet with much temporary defeat and, perhaps some failures. When defeat overtakes a man, the easiest and the most logical thing to do is to quit. That's exactly what the majority of men do.

This quote is saying that, before a person is successful, they are likely to experience a lot of temporary failure and maybe even some long-term setbacks. When these failures occur, the most logical thing to do would be to give up. Unfortunately, this is what most people actually do.