It means that you can see at 20 feet what a person with normal vision can see at 20 feet. We measure vision using a Snellen chart at 20 feet. Depending on which line you can see in the chart, your vision can be 20/20, 20/40, 20/200 etc. If your vision is 20/40 it means that you can see at 20 feet what a person with normal vision can see at 40 feet.
Your vision needs to be 20/40 or better in order to obtain a driver's license, and less than 20/200 is considered legally blind.
When we write you a prescription for glasses, the numbers just indicate the power of the lenses that you need to achieve your best corrected vision. They do not state the level of vision that you achieve with your glasses. Most people can see 20/20 with their glasses, but some ocular conditions like cataract can prevent you from seeing 20/20, even with glasses.