This quote means that people who don't know that something can't be done are often the ones who succeed. This is because they don't have the same mental barriers as those who believe that something is impossible. They are willing to try new things and take risks, and this often leads to success.
For example, if someone wants to start their own business, but they believe that it's impossible because they don't have any money, they will never even try. However, if someone else has the same idea, but they don't believe that it's impossible, they will be more likely to take the necessary steps to start their business. And who knows, they might just succeed.
This quote inspires people not to give up on their dreams, even when they seem impossible. It encourages people to believe in themselves and their abilities, and to take risks in order to achieve their goals.