Gasless or meta transactions are transactions which have had their gas cost abstracted and thus paid for by a different party than the user. It allows for applications to subsidise a users gas costs allowing for a seamless experience, particularly for users unfamiliar to the crypto ecosystem. There are primarily two ways to implement meta transactions, on your smart contract itself or via a proxy/smart contract wallet.
There are many open source implementations of smart contract wallets (Argent, Authereum, Gnosis Safe) and standards for implementing them on your contracts (EIP-1613) that you can use or take reference to.
These contracts can be complicated, and take some time to deploy/use. We provide a smart contract wallet deployment tool kit that can get you setup quickly, which applications are free to use.