Every trade is classified by whether it lifted the offer or hit the bid. Delta is buys minus sells for a period; CVD is the running sum across periods. It measures intent, because crossing the spread is what someone does when they want to trade now rather than wait.
Its value is in the mismatch with price. Price grinding to new highs while CVD flattens or falls means the advance is being carried by passive buyers absorbing supply rather than by aggressive demand, which is a fragile way to reach a high.
The measure is only as good as the trade classification behind it. A true CVD needs tick-level data with the aggressor side marked; anything reconstructed from candle data is an approximation, and the two can disagree materially at exactly the moments that matter.