You basically start talking to the Mayor of the Magikarp village and from there, you are guided as the player to how you train, care, and battle your Magikarp in jumping contests! But you are limited but luckily, as you grow with each and every match, you get EXP for your trainer and level up your way to getting stronger level Magikarp. They go in retirement if you level them up fully by training them and go for the final stretch in the leagues. Magikarps can and will die, so assess the risks when you are presented opportunities. It is a mobile app game by The Pokemon Company
I suggest downloading the game and playing it yourself.
Just try it.