It is just a parameter that stands for the vision condition of your eyes. And we use 20/20 to express the best vision. And 20/30, 20/30 are not so good as 20/20. So, you may have a mild nearsightedness. If you feel vision blur when you want to focus something, you need buy prescription eyeglasses that can help see thing clear. Remember to buy prescription eyeglasses according to your vision requirements.
People who have 20/20 vision can read the eye chart when they stand 20 feet away from the eye chart. While people who have 20/40 vision should stand closer to the eye chart so that they can read the eye chart completely. If you have 20/40 vision, it means that when you stand 20 feet away from the object, you can see what someone with good sight would see when they stand 40 feet away from the object. That is to say, your vision is worse than normal and need glasses to help you see better.