A neutral oil is a cooking oil with little to no flavor, aroma, or color of its own, used as an unobtrusive base for frying, sautéing, baking, dressings, and sauces. Common examples include canola, corn, soybean, sunflower, grapeseed, and refined peanut or avocado oil.
Neutral oils are usually refined to strip out the compounds that carry flavor, but 'neutral' and 'refined' aren't interchangeable: some refined oils (refined peanut oil, for instance) still carry a mild trace of their source flavor, and a few minimally processed oils, such as rice bran oil, taste close to neutral without heavy refining.
Because they don't compete with other ingredients, neutral oils are the default choice when a high smoke point is needed or when another ingredient should be the star, and they're often blended with a small amount of a more assertive oil, such as toasted sesame or extra-virgin olive oil, for accent.