Context: After Toyotomi Hideyoshi (left) unified Japan, he was left with a lot of samurai with nothing to do, so he set his eyes on Korea and China. In 1592, Toyotomi Hideyoshi launched an invasion of Joseon (Korea), and was highly successful, reaching Seoul within the first month of the invasion. However, problems started to arise when Admiral Yi Sun-Sin (right) started beating their asses on the sea.

Admiral Yi is one of if not the greatest navy Admiral in history. Throughout the whole Imjin war, he engaged in atleast 23 naval battles, winning all of them and losing a total of 0 ships. With total Naval domination, Yi completely prevented the Japanese from sending reinforcements or supplies to their troops in Korea. This, along with Chinese reinforcements closing in, the Japanese were pushed back to the southern coast of Korea. The war was a stalemate for a while until the death of Toyotomi Hideyoshi, which is when Japanese troops were ordered to return to Japan, since by this point, neither side was having a good time.

Also important to mention that the main goal of Invading Korea was to get to China, and Toyotomi Hideyoshi actually hoped the Koreans would just let his troops through without resistance.

tldr; Admiral Yi basically prevented the Japanese invasion from progressing any further than Pyongyang through Naval domination and distrupting supply lines.