Every futures trade has a buyer and a seller, so what happens to open-interest depends on whether each side is opening or closing. New buyer plus new seller adds one to open interest. Closing buyer plus closing seller subtracts one. A new participant taking over an old one's position leaves it unchanged. volume rises in all three cases.
Read together they describe conviction. Price up on rising volume and rising open interest means fresh money is going long. Price up on rising volume and falling open interest means shorts are covering, which is a weaker signal because the buying stops when the shorts are done.
Open interest is also the practical guide to where to trade. During a roll, the month with the larger open interest is where hedgers live and where your resting orders will fill.
Example: corn trades 400,000 contracts and open interest rises from 1.42 million to 1.46 million. Of the day's volume, roughly 40,000 contracts were new positions and the remaining 360,000 were traders swapping existing exposure.
Related: open-interest, volume, roll-date, commitments-of-traders, front-month