When someone says, "I know you're doing well for yourself," they are acknowledging and complimenting your success and achievements. It means they believe you are doing good or even thriving in various aspects of your life, whether it's professionally, personally, or both.
This phrase typically implies that the person recognizes your hard work, accomplishments, and possibly the positive changes you've made in your life. It can be a way for them to show admiration, respect, or support for your endeavors. It also suggests that they have a positive impression of your current situation and see you as someone who is successful or on a positive path in life.
It's important to take such a compliment gracefully and modestly, acknowledging the efforts you've put in to achieve your goals while staying humble about your accomplishments.