Chargestone Cave is a cave which is literally charged with energy, causing the rocks within to levitate. It connects Route 6 to Mistralton City. When leveled up here, Nosepass and Magneton will evolve.
You won't be able to pass through the cave until you've beaten Clay at the Driftveil City Gym, as his Pokémon will smash the Galvantula web blocking the entrance.
When you first enter the cave, you'll meet the Shadow Triad. They lead you to N, who says that Team Plasma are waiting to test you. Afterwards, Professor Juniper arrives with Bianca in tow acting as her bodyguard. The Professor reveals she's here to study the Klink in the cave and gives you a Lucky Egg.
After you've travelled through the cave you'll find N near the exit and, as usual, he wants to battle. Once you've defeated him, Professor Juniper and Bianca come along, much to N's disgust. He appears to have a problem with Juniper due to her research, and leaves the cave. The others exit soon after, and you're free to continue your journey.