You know how in school, you learn all sorts of things like how to read, write, do math, and learn about all sorts of topics? Well, as you go through school, you can get different levels of education. The more education you get, the more you know and the more things you can do.
For example, when you first start school, you might learn how to read simple words like "cat" or "dog." But as you go to school for more years, you learn bigger words and more complicated things to read and understand.
When people talk about educational attainment, they are talking about how much education someone has. So someone who finished high school has a different level of educational attainment than someone who stopped going to school after elementary school. And someone who went to college and got a degree has an even higher level of educational attainment.
People who have higher levels of educational attainment can often get better jobs and make more money than people with lower levels of education. And they can also understand and do more things in their lives because they know more.