If you lack something, you need or want something that's missing. When there's a lack of food, people starve.
There are a lot of things lacking in the world. If you have no video games, then you're lacking them. People who are thirsty and starving lack food and water. Many people lack money and health care. A lack of something is the opposite of an abundance of something. If you have plenty of books, then you don't lack them — though you may lack enough time to read them all.