“1K”, “2DK”, “3LDK”..etc are examples of how Japanese describe the floor-plans of apartments/houses.
The number indicates the number of rooms.
L = Living Room, D = Dining Room, K = Kitchen.
E.g. The layout on the left shows a typical 1LDK layout and on the right a typical 2DK layout.
Most LDK rooms has a bigger living room and kitchen is separated, while DK rooms have smaller area and kitchen is not separated.
Difference Between 1K and 1DK
As you can see for 1K or 1R(Room), there is only a little space for the kitchen since there is no Dining area. While 1DK room has a separated room and a wider area in the kitchen.