The human eye is like a giant magnifying glasses. It has a power of about +60 diopters. If the eye has too much curvature or to little curvature for its size that is when someone will need glasses. Here is a breakdown of Myopia, Hyperopia and Astigmatism and what they mean.
Myopia: Aka nearsighted. Means you are able to see up close and need glasses to see far. People who are nearsighted have prescriptions with a negative sign in front of them. Essentially their eye has to much power.
Hyperopia: aka farsighted. Means you are able to see far away but your eye has to focus to see up close. When we are younger our eye is able to naturally focus and many people who are farsighted do not need glasses, but as we get older or spend more time on screens and devices our eyes have a hard time focusing and we need glasses for close and eventually need them for both far and near. People who are farsighted have prescriptions with a positive sign in front of them.
Astigmatism: Is when your eye has two different curvatures and two different powers. The best analogy is: your eye is shaped more like a football (with one flat and one steep curve) instead of a soccer ball. Astigmatism can affect vision for both far and near and is usually more bothersome at night and in low light conditions. This is due to the fact that our eye lets more light in and more of your eye curvature is used and thus more glare. People with astigmatism usually notice a shadow when looking at images and screens.
I hope this helps you better understand your prescription!