Before Hangul was created, people wrote using Hanzi (Chinese characters). 
Hanzi is logograms, meaning they represent meanings rather than sounds.
However, since the Korean language is completely different from Chinese.
To write something that a Korean says,
it had to be translated into Chinese first, and then,
when reading it, it had to be translated back into Korean.
Because of this, only a small Aristocratic class
who had the time and resources to study Chinese characters could read and write,
making it nearly impossible for commoners to learn how to write.

King Sejong was deeply concerned about this
and created a writing system that would be easy to learn and use.

Learning Hangul is very simple.
Jeong In-ji, the Minister of Rites, said,
“A wise person can learn it in half a day, and even a fool can learn and use it in ten days.”
In fact, children in Korea typically begin reading and writing at the age of three.