Pokémon Magikarp Jump is a fairly simple game with two major modes of gameplay.
The main mode you'll see in the game is where you have your Magikarp in the water. Here, you have the ability to feed your Magikarp, train your Magikarp and interact with the majority of the game including the shop and the stat features. To feed a Magikarp, simply tap the berries and it'll eat them. Each bit of Food has its own unique properties and the high rated berries are rarer to appear. They will continue to regenerate over time.
Training Mode is also found here where you can send your Magikarp to train in a random method. This will give your Magikarp motivation and experience to grow. Tap the screen repeatedly to improve your score and make sure you get as many points as possible.
You also can access the Shop in this mode.If your Magikarp has reached its maximum level, you cannot access this part of the game and you have no choice but to go into the League mode.
When you have raised your Magikarp a significant amount, you can send it into the league. There are six different leagues all with multiple levels for you to play through. In each level, you face off against a Magikarp and whether you defeat it or not depends solely on your Magikarp's motivation compared to theirs. If yours is higher then you win.
However, if you lose the matchup you may not have enough stamina left, you will be unable to use the Magikarp again and will need to fish a new Magikarp and begin training it from scratch. However, based on the performance of your last Magikarp, you will receive experience points to level your trainer up which will allow for you to get higher levelled Magikarp, each with unique features. If your Magikarp does have stamina to continue but lost a battle, you need to use a LP Spray to do the next level, or wait 1 hour and 30 minutes