King Sejong realized that Chinese characters with which Korean was originally written took too long to learn and didn’t perfectly fit his language. He wanted everyone to be able to read, not just the ruling class. In the 15th century he invented the Hangul alphabet that has been called the most scientific alphabet in the world.
Each consonant letter is the shape that the tongue, teeth, lips, or throat forms when making the sound. Hangul was not embraced at the time nor widely used until the mid-20th century. But once it was adopted and accepted, many were able to learn to read and write and the literacy rate in Korea skyrocketed.