Like all games with tiles or cards, Rummikub is a game of both luck and skill. The degree to which the game is played with skill is largely determined by the players. For the beginner the game can be played with a lot of luck with each player drawing and melding as the tiles come up with no particular strategy. Basically they are allowing chance to determine their success during any given hand. This way can be a great deal of fun and still bring on lively games, but it is not very competitive.
With a player who is more competitive it becomes important to maximize the chances of winning by playing the most strategic way that you can. With 106 tiles available there is an exorbitant amount of possible rack combinations. In fact the combinations range into the trillions. However, most of these are useless when it comes to melding, therefore you need to play the rack and table wisely to maximize your ability to meld all of your tiles.
With this many possibilities, there is still a huge amount of luck involved for every player at your table. However, you do not want to live on just luck alone. The difference between the beginner and the expert player is that the good player takes advantage of all opportunities to improve their chances of winning. Even if the good player is on a streak of extreme bad luck, they will still lose less often than the random player. If luck is on the good side for the better player, they will win with much larger scores than the random player.
When you are given your starting tiles, it is basically chance on what tiles you actually get. The key to being a good player is to be able to arrange the tiles to very specific melds that are needed to lay them on the table in a way that works for you later on. This can be done by exchanging them for other tiles. The other tiles must come from either the discard of the previous player or from the drawing pile. There are only certain tiles on any given play that are advantageous to you. It is important to know what your chances are of getting these tiles. For this reason, you need to try to figure out the location of as many of these tiles as possible. At the beginning of each hand, the only ones that are going to be known to you are the tiles on your rack, but as the game progresses there will be clues which you need to pick up on that tells you where the remaining tiles are.
This is what is going to differentiate a player that plays his game on random chance and luck, and the player that is using intelligent strategy to win.