It's hard to explain it further without you having knowledge of the game. But basically......as you get better in the game, you encounter rarer types of pokemon that you don't encounter when you're just starting out playing.
Additionally, the "common" pokemon that you encounter are much better when you're a higher level player, meaning the pokemon you catch are stronger and can do better in battle (the ultimate point of the game is to battle with the pokemon you catch).
Like I said, it's sorta hard to explain it without the person have a working knowledge of the pokemon universe (it was a Gameboy game back in the late 90s that my generation was crazy over). It would be like you trying to explain the finer points of the pick-and-roll to someone who doesn't even know what basketball is.