Let's talk about the Experience Share. The Experience Share (Exp. Share for short) is a simple tool, present in every main series Pokémon game since 1999's Pokémon Gold and Silver, that shares the experience one Pokémon gains from defeating others in battle with the others in your party. It's useful for a few things, but the big one is your experience of the game itself: the Exp. Share is the single most influential tool you have for manipulating the difficulty of the game, helping your whole team level up faster with it on, or slower with it off.
Over the years, as public tastes have changed - and main series developer Game Freak's with them - it's become kinder, moving from sharing experience with just a single other Pokémon to with your whole party, then experience itself changed from being earned just via battles to all successful catches of wild Pokémon, too. And then finally, most generously and most dramatically, with the arrival of Pokémon Brilliant Diamond and Shining Pearl it's also now been locked to "on" for the entirety of generation eight.