Damage counters are placed on a Pokémon's damage counter box, which is located on the bottom-left corner of the card. The damage counter box can hold up to 10 damage counters, and when it is full, the Pokémon is knocked out.
Damage counters can be placed on a Pokémon by a variety of attacks. Some attacks do a specific amount of damage, while others do damage based on the attacking Pokémon's stats. There are also some attacks that can do indirect damage, such as by poisoning or burning the opponent's Pokémon.
When a Pokémon is knocked out, it is placed in the discard pile. The player who knocked out the Pokémon then draws a Prize card. Prize cards can be used to power up your Pokémon or to draw more cards from your deck.
Damage counters are an important part of the Pokémon Trading Card Game, as they help to keep track of the progress of the game and determine which Pokémon are still in play.