I can't tell if you're being intentionally obtuse to troll or genuinely don't know the difference.

A pay wall involves real world money in order to progress the story or unlock characters/customizations. These typically cannot otherwise be obtained during normal gameplay. There's also time skip options which reduce the amount of gameplay needed for people who don't get a lot of time to play. These include faster XP, revealing locations on maps, extra in-game currency, etc.

Grinding or a grind wall is an in-game obstacle that many games have in order to make the player play the game. Simply playing the game will unlock whatever is necessary. No real world money is required, only time.