This is the guiding pole for the subway station.
As you can see, in the circle above there are line number(6호선), and station number 639. And under the circle, there is the name of the station in Korean 한글, English, and Chinese Characters.
The last number in the yellow block is the exit number of the station. Normally you can find the way on the map at the station by this exit number.
If you visit Seoul, you can go almost everywhere by subway and bus.