When someone says, "I know you're doing well for yourself," it means that they believe you are experiencing success and doing relatively well in various aspects of your life. The phrase implies that the person perceives you as being prosperous, achieving your goals, or making positive advancements in your personal or professional life.
This expression is often used as a compliment or a way to acknowledge someone's accomplishments and achievements. It indicates that the person speaking admires or respects your progress and is pleased with the direction your life is taking.