O que é Imbalance volume, imbalance price?

Example 6.4 (A wind farm’s afternoon)

A wind farm sells 80, 90, 100 and 100 MWh day-ahead for four hours at 62, 58, 55 and $57\,\mathrm{EUR}/\mathrm{MWh}$. Before the last hour its forecast falls by 30 MWh and it buys 30 back at $170\,\mathrm{EUR}/\mathrm{MWh}$ intraday. It meters 82, 88, 101 and 55 MWh; the imbalance prices are 60, 75, 50 and $240\,\mathrm{EUR}/\mathrm{MWh}$. Its trading revenue is EUR 16 280 (the day-ahead sales less the buy-back), and its imbalances ($+2$, $-2$, $+1$ and $-15$ MWh) cost it EUR 3 580, nearly all in the last hour, when it was short while the system was short too: EUR 12 700 in all.