Open interest is an indicator of the liquidity of a particular option indicating the amount of options bought/sold at a certain expiration date and strike.
When it comes to options as other fixed-term contracts, records are kept of open positions expressed in the so-called open interest. The trade volume is fixed every day. This is one of the most important indicators in the options market since option contracts market liquidity is determined according to it. The spread between the supply and demand prices of various options depends on the existing level of liquidity.
Open interest does not show the trade volume, but determines the number of options contracts that are currently open. When a new position is opened, open interest increases by the number of open contracts but if the previously open positions are closed, this leads to a corresponding decrease in open interest.