Stablecoin issuance is the process by which new units of a stablecoin are created and put into circulation. For fiat-backed coins it usually happens when an approved customer sends money to the issuer, which adds it to the reserves and creates the same value of tokens on a blockchain. Creation and its reverse, destruction on redemption, are explained in full in Stablecoin Minting and Burning.
The word issuance also covers the business and legal side: the stablecoin issuer is the company that promises to redeem the coins and is regulated for doing so. Under rules such as the EU's MiCA, issuers need authorisation and must hold reserves that match the coins in circulation. Supply shrinks again through redemption, when holders hand coins back for money.