Volume tells you how much traded; open interest change tells you what it meant. Volume with rising open interest means new positions were established. Volume with falling open interest means existing positions were closed. The same 10,000 contracts can be a new bet or an exit.
The figure arrives with a lag, after the clearing house processes the day, and it is a net number — 3,000 opened against 2,800 closed shows as +200. It remains the most reliable public signal in a field full of unreliable ones.
Example: XYZ $55 calls trade 12,000 contracts against 1,400 open-interest. If tomorrow's open interest reads 12,800, someone built a large new position. If it reads 1,600, the day was mostly the same contracts changing hands intraday.
Related: open-interest, option-volume, volume-open-interest-ratio, unusual-options-activity