Island tiles are the hexagonal pieces placed on the board to represent parts of the island. They can be placed in any complete hexagonal space on the board. (The incomplete spaces at the top and right borders of the board are not usable.)
Island spaces are the spaces on the board where tiles can be placed. A tile is not a space. If a game effect instructs you to place something on a tile, you can’t place it on a space, and vice versa.