Many games have a system where you can purchase items by using in-game currency. This currency can be earned by performing certain actions, such as
grinding and sometimes it can be purchased with real money, which makes it a
premium currency. If earning the in-game currency through gameplay is so slow that it's almost impossible to realistically earn enough to buy the necessary items, then spending real money becomes necessary. The players who spend the most will have the biggest advantage and a free-to-play (F2P) player will be at a significant disadvantage.
If you can spend real money to give yourself an advantage over other players, then a game is exhibiting the "Pay to Win" dark pattern. In addition to accelerating the normal earning of in-game currency, you may also be able to purchase a "cheat" or unlock some otherwise unobtainable item that gives you an advantage.
Sometimes games give large incentives to be in the top ranks, but make it impossible to get into the top ranks unless you constantly spend money to purchase the latest items or upgrades. This dark pattern is sometimes called "Monetized Rivalries", and when its combined with another dark pattern, called
depreciation, it can setup a player to be constantly needing to spend money to stay ahead of other players.