Quick definition that we use to describe map terrian that does not flow, and has jagged edges and sudden change in transition which reminds the individual looking upon the terrain that they are viewing a square based tile system instead of isometical beauty. It is undesirable to have blockyness within your map.
A common technique to get rid of it is to "hide" it with sprites. Sprites, being doodads and other such things which look as if it were part of the map and hide ugly terrain blocks under it meanwhile.
The disadvantage of course, is that too many sprites on your map can cause problems, such as units stop firing. Depending on the numbe rof units that can be firing at one time during gameplay, you may want to limit this. If its an RPG, you can use more sprites seeing how the player usually only owns one unit. In a melee or massing map, you'd best restrict yourself under 100 sprites.