From StrategyWiki, the video game walkthrough and strategy guide wiki

This walkthrough will contain information about the maps and sections within Psychonauts required to complete the game.

Most of the tasks in the game involve collecting psychic energy from figments, baggage, vaults and cobwebs. As you collect them, your psychic power improves, increasing in strength and quality. While you are not required to collect all of them, you only need enough to obtain rank 30 (required for invisibility).