If you’re not familiar with how Pokemon battles are done, there are usually two Pokemon trainers, each has six Pokemon in hand. Each trainer will select one Pokemon to battle the other trainer’s Pokemon. The goal is to make the other Pokemon lose all of its health points (HP) by using different moves (for example, Thunderbolt or Hydro Pump). Once the Pokemon loses all of its HP, they cannot be used again.
The trainer can also switch out their Pokemon. This is usually done to take advantage of type-effectiveness (for example, fire type Pokemon is effective on grass types, but is weak against water types).
In the middle of the battle, the trainer can also bring back the lost HP of their Pokemon by using healing items (for example, potion). There are also items for healing status conditions (for example, sleep or paralysis). These status conditions are invoked by using specific moves like Sleep Powder or Thunder Wave.
The first trainer who loses all of their Pokemon loses the game, while the trainer who still has Pokemon wins.