Where a normal volume bar shows volume per time, a volume profile shows volume per price. Its peak is the point-of-control, and the area holding about 70% of the volume is the value-area.
High-volume nodes are prices where the market agreed on value and tend to act as support and resistance. Low-volume nodes are prices the market rejected quickly, and price often moves through them fast.
Example: over the last 20 days, 35% of a stock's volume traded between $48 and $50 and almost none between $52 and $54. A move into $52 could travel to $54 quickly; a drop into $49 is likely to find buyers.
Related: point-of-control, value-area, volume, support