Summary
An Overlay is a study drawn on top of the price chart. It uses the same time axis as the price series so you can compare the study with price movement.
Available overlays
Moving Average adds a smoothed line based on a selected averaging method and period. Bollinger Bands add a moving-average centre line with upper and lower bands based on a selected period and standard-deviation multiplier.
Parameters
For Moving Average, Type selects SMA or EMA and Period controls how many observations are included. For Bollinger Bands, Period controls the rolling window and Std Dev controls the distance of the outer bands from the centre line.
Things to keep in mind
Overlay settings change the appearance and responsiveness of the study. They do not change the underlying price data or create a prediction of future performance.