From Traderpedia

Definition: The total number of outstanding futures or options contracts that have not been exercised, expired, or delivered at the end of the day.

[edit] Interpretation

In an uptrend when Open Interest increases this is considered to be bullish. Money is flowing into the market because long and short positions are being opened, so the trend is more likely to continue.

The opposite is true in a downtrend, when it is bullish to see Open Interest decrease.