It refers, for a given collateral/stablecoin pair, to the sum of the amount of stablecoins hedged by HAs. If there is one HA hedging 2 wETH from the Core module that entered when 1 ETH was worth 2000โฌ, then this HA covers 2 * 2000 = 4000โฌ of stablecoins for the Core module, and the total hedge amount by HAs for the wETH collateral is 4000 agEUR.