Open interest is a measurement of the outstanding open positions in a derivative security. Strong open interest means the derivative will have high liquidity.
Open Interest is not the same thing as Trading Volume, but it does give an indication of liquidity and activity in a derivative. Open Interest is the number of open positions for a derivative, like an option.
The Options Clearing Corporation tallies up the ‘open interest’ numbers, but they are not posted until the morning following the count. Open Interest isn't necessarily indicative of a bullish or bearish forecast for the underlying security, but it does generally mean that the option will have high liquidity and that a seller will be able to find a buyer.