The Nifty Dictionary: Lazy Minting
While gas is a critical feature of Ethereum, high gas fees can disincentivize people from creating NFTs at all. Minting fees can be especially daunting for first-time NFT creators, who may not know whether anyone will be willing to pay for their work.
Lazy minting works by essentially creating the metadata for the NFT, without actually creating an instance of the NFT itself. Once someone purchases that nifty, it is then officially minted, and a gas fee is incurred. Typically, the fee is rolled in to the buyer's cost.
Note that just because some platforms support lazy minting, the costs of creating and selling niftys may still not be entirely free. On OpenSea for example, the seller must pay an upfront network fee to create a "collection" before they will be able to mint an NFT, lazily or otherwise.