Volume delta is taker buy volume minus taker sell volume. If 100 BTC was bought by takers and 60 BTC sold, delta is +40. Cumulative volume delta, or CVD, adds those figures up over time, so it rises while takers are net buying and falls while they're net selling.
Divergences
Most of the time price and CVD move together. The moments worth noticing are when they don't. Price rising while CVD falls means passive sellers are being lifted less and less, or that price is being held up by something other than buying pressure. Price holding flat while CVD plunges can mean large resting bids are absorbing the selling.
Spot versus perpetual
CVD on spot markets and on perpetual futures can tell different stories. A rally driven by perpetuals with weak spot demand is built on leverage and is more likely to reverse quickly. A rally that spot buyers are driving is generally on firmer ground.
The venue problem
Every venue has its own flow, and no single CVD covers the whole market. The figure on one exchange can look very different from another's, so compare venues before reading too much into one line.