끼다: to cloud over, or hang around
The fog in this sentence is hanging around, so it is misty.
Also, this is the grammar pattern (blunt form + it-da (sorry, no Keyboard on this computer))which indicates the current condition of something. So, as opposed to "It is getting foggy" this is more like "It is currently foggy". Another example would be the difference between "The door is opening / The door opens" and "The door is currently open".
The English translation says "misty" even though the Korean word for fog is being used. Mist and fog aren't the same in English. The translation should be "it is foggy".