Thought is the original source of all wealth, all success, all material gain, all great discoveries and inventions, and of all achievement.

This quote means that all wealth, success, material gain, discoveries, inventions, and achievements come from thinking. Thinking about ideas is the first step for any kind of progress. For example, to start a business all you need is a good idea. To invent something you need to think of how to make that invention work. To make a great discovery, you must think of the possibilities. Thinking is the fundamental source of all these advances. Without thinking, none of this progress would be possible.