The lower line is the "Average Volume" based on lifetime history. Anytime volume is below average, this likely denotes most people have capitulated or the markets have grown bored at these price ranges. A squeeze in volume primes the asset for a big move.
The upper line is the 3rd standard of Deviation from the average line. Anytime volume touches that line or goes higher, this denotes its a very big move relative to how the asset typically trades. If the volume stays above this range for an extended period of time, it would be wise to watch the chart closely and start taking profits off the table whether you're shorting or longing the asset.