The Exp. All (Experience All) is an item in the first generation of Pokémon games, available in Pokémon Red, Blue, and Yellow. You can pick up the item from one of Prof. Oak's Aides in the gate right to the right of Fuchsia City after obtaining at least 50 species of Pokémon in your Pokédex. It's an item that you can't sell, but you're able to place it in your PC if you wish. Now, you might be asking, what good is the Exp. All? Well, not only does it do what you'd think it does, give experience to everyone in your party after a battle, it shares it equally. But there are a few things to note.
The Exp. All gives half of the experience gained in a battle to any of the Pokémon that took part in the battle, then the other half is split between every Pokémon in the party, even the ones that didn't battle. So, for example, if I had 6 Pokémon in my party and battled a Pokémon that gave out 1,200 experience using two of them, the two Pokémon that battled would each receive 300 experience, which added up totals 600, or half of the original 1,200 experience. The rest of the 600 experience gets shared between the 6 in the party. So every Pokémon would then get 100 experience, and the two that battled would receive a total of 400 experience. I personally recommend only having two or three Pokémon in your party so the experience gets more evenly distributed throughout the ones you're training.
And that's just about all there is to the Exp. All! Enjoy the help on training!