In the Pokémon games the player can capture a Pokémon he/she likes to have in his/her team, but mostly the catch doesn't succeed as they hope it should be. Each Pokémon has a catch rate. It depends on the Pokémon HP and the type of Poké Ball the player uses.
The lowest catch rate is 3, which belongs to the Beldum family and mostly almost every legendary Pokémon in the game. That low catch rate is n example to say that the Pokémon is very hard to catch which that you must probably use over 20x PokéBalls or even more because the highest catch rate is 255 which belongs to small Pokémons such as a Caterpie or a Pidgey which can be easily caught by a Poké Ball without damaging them.
Depending on the amount of HP, current status ailment, level of the Pokemon, and/or the Poké Ball being used, the chances may raise or lower. Because of this, the player may sometimes have to use stronger Poké Balls or more of the same one to catch the desired Pokémon. If an attempt to catch a Pokemon fails too many times, the Pokémon may flee.