The indicator is the face-up tile that decides which tile becomes the okey (joker) for that hand.

After the tiles are dealt, one tile is turned face-up as the indicator. The okey is the tile one number higher than the indicator, in the same colour. For example, if the indicator is a red 5, the okey is the red 6. If the indicator is a 13, the okey wraps around to the 1 of that colour. The indicator stays visible for the whole hand, so it is a constant reference for which tile is your joker.