No man has a right to expect to succeed in life unless he understands his business, and nobody can understand his business thoroughly unless he learns it by personal application and experience.

This quote means that if you want to be successful in life, you must understand what your job or business entails. You cannot understand it completely unless you put in the work and gain direct experience in the field. In other words, you cannot expect success if you do not dedicate your time and effort to understand your job.