An unknown entity, often referred to as 'God', decides to host a game called Dragon Ball War instead of the Holy Grail War. The system, though quite similar to the original setting, involves Servants battling other Servants for their Masters. However, they must now obtain Dragon Balls, small beads, from other Masters/Servants. Possessing all seven Dragon Balls could grant the Master/Servant any wish they desire.
Masters are chosen randomly by the Dragon Balls or, in some cases, by the Servants themselves. Additionally, each Master receives all the pertinent information regarding the game, such as the nature of the Dragon Balls, their capabilities, the role of Servants, and how to summon them using catalysts, among other things. In short, every Master gains relevant knowledge of the War the moment they obtain a Dragon Ball.
You can ask me further in the comments.