Open interest refers to the number of futures or options contracts that are not delivered or closed on a particular day.
In other words, this is the number of contracts that have been traded but have not yet been executed by an offsetting trade or an assignment or exercise. This will also be the number of buy market orders before the stock market opens on a given day. The open interest number for a given stock will change if there are an uneven number of options of a particular underlying asset being bought or sold throughout the day.
If the number of shares being bought is equal to the number of shares being sold, the open interest will be 0.