The coin bears a Dangun date, with a calendar starting at the legendary founding of Korea in 2333BC.
On the reverse of the coin you can see a turtle ship, also known as Geobukseon, a type of large Korean warship it was used to fight against invading Japanese naval ships. This design is often recognized as the first armored ship in the world.
These coins are not very valuable. It is probably one US dollar or so in worn conditions and 5 to 7 US dollars in uncirculated conditions. Coins dated 4292 (equivalent to 1959AD) are a little more valuable that coins dated 4294 (equivalent to 1961AD), but only in uncirculated condition.