Definition
Every contract open is one long and one short, so the figure is not the two sides added together: it is what the market is holding, counted once. Hyperliquid publishes it per market, as the openInterest of each perpetual's asset context, in units of the asset, which this site converts to dollars at the mark price so markets of different prices can be compared.
It is a stock, not a flow, and that is what separates it from volume. Volume counts what traded over a window and starts again at zero; open interest is what is still held at this instant. It rises when a new long meets a new short, falls when two holders close against each other, and does not move when a position merely changes hands.
Hyperliquid holds an open interest cap per market, publishes the list of markets sitting at theirs, and names the orders it refuses for that reason: positionIncreaseAtOpenInterestCapRejected and openInterestIncreaseRejected. What the figure never says is the side: it is symmetric by construction. It also says nothing about the margin behind those positions, so two markets with the same open interest can be one held at 2x and one at 20x.
Example
An example, in round figures. A market holds 1,000 BTC of open interest, which at a mark price of $100,000 is $100 million. A trader opening a long of 10 BTC against another trader opening a short of 10 BTC takes it to 1,010 BTC. If instead that long is sold to someone who is opening a long, the contract survives with a new holder and the figure stays at 1,000 BTC.