For installation of the channel of the linear regression on the diagram, you should select the "insertion" - "channels" menu item - "The linear regression"
The channel of the Linear Regression is the standard tool in any science, which allows to find the true dependence of one parameter to another - in our case dependence of the price from the time.
The channel is built on the basis of the prices which you have specified in settings.
The middle line in the channel of the linear regression is also an "average square" or as it is also called the "equilibrium" price and the deviation from it, should be understood as a separate "accidental" fluctuation connected to not typical action of a buyer and a seller.
Two side lines are at the same distance from the central line and represent the maximum deviation from the equilibrium price.
The channel of the linear regression is a recognized tool in all exact sciences, and it will help you to understand better a situation in the market.