What it means
High-volume prices act like magnets; low-volume prices are crossed quickly.
In practice
The profile shows where trading actually took place. Prices with heavy volume are areas of agreement where the market spent time, and they tend to slow price down. Thin areas were rejected quickly and are crossed quickly when price returns.
Traders use the point of control and the edges of the value area as levels, and treat a move out of the value area with volume as the start of a possible trend.
What to watch for
It needs real volume data, so it is meaningful on futures and stocks and unreliable on spot forex. The picture also depends on the period chosen: a daily profile and a monthly one highlight different levels.