Open interest counts contracts that have been opened and not yet closed, unlike volume which counts everything traded in a period. Rising open interest means new positions are being built; falling open interest means positions are being closed.
Combined with price it describes the character of a move. Price up with open interest up is new money buying. Price up with open interest down is shorts covering, which is a weaker foundation because that buying pressure ends once the shorts are out.
In crypto perpetual futures it is the clearest gauge of fragility. High open interest with a stretched funding rate means crowded one-sided leverage, and those conditions precede the cascade liquidations that move price several per cent in minutes.