The _uri is the base URI that is set in the constructor when the contract is first deployed. In this case it is the 'ipfs.io' gateway but really it can be whatever you set it as in your own contract.

Check lines 30-37 in the openzeppelin erc1155 code: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC1155/ERC1155.sol

Hope this helps!