Equivolume plots price range as bar height and volume as bar width. A wide, short box means heavy trading that went nowhere, a sign of two-sided conflict. A wide, tall box means a decisive move on heavy participation. A narrow tall box means a move on little volume, which is easier to reverse.
The idea is simply to put volume into the same visual field as price instead of a separate pane, so you cannot ignore it.
It is a presentation choice, not an indicator. Modern platforms rarely support it well, and the same reads are available by watching the volume histogram or using relative-volume. Worth knowing as a concept: participation gives a move its weight.
Related: volume, relative-volume, volume-bars