“there is only one thing that makes a dream impossible to achieve: the fear of failure.” meaning

It means that if you fear failure you're most likely going to focus on nothing else but failing. Oftentimes you will fail at first, but thats what makes a dream significant in life. It's definitely not always easy accomplishing your dream. But if you never try because you don't want to fail, you're never going to be able to grasp your dream in your hands.