Opening means laying down melds whose tile values add up to at least 101 points before you are allowed to play tiles onto the table.

Each tile is worth its number value, and the okey and blanks take the value of the tile they represent. You must reach the 101 threshold with valid runs and sets. Until you open, your tiles stay on your rack and you cannot lay off. The requirement is the heart of the game, since you must balance collecting enough points to open against finishing quickly.