It is determined by the changes in price and volume. The volume acts as a weighting coefficient at the change of price — the higher the coefficient (the volume) is the greater the contribution of the price change (for this period of time) will be in the value of the indicator. The indicator is also known as Accumulation / Distribution Index.
Common known variants of this type of indicator are the On Balance Volume, Chaikin Oscillator or Chaikin Money Flow.