A feature that allows you to have a visual reference for the probability that a market will be trading within a certain price in the future.
For example, on this chart of TSLA, a probability envelope of 68% or one standard deviation (default setting) is illustrated. The interpretation is that throughout the lengths and width of the Envelope, the market has a 68% probability of prices staying within this range. As time increases so does the price range that the market has the probability of reaching.