IPFS is the Inter-Planetary File System. curl supports IPFS only via an HTTP gateway. It means that it understands IPFS URLs when given to it, but you must also provide a working gateway URL for curl to use to retrieve the content. curl does not speak IPFS natively.
The --ipfs-gateway lets the user specify the IPFS HTTP gateway URL. Like this:
curl --ipfs-gateway http://localhost:8080 ipfs://bafybeigagd5nmnn2iys2f3d/
If you opt to go for a remote gateway you should be aware that you completely trust the gateway. This is fine in local gateways as you host it yourself. With remote gateways there could potentially be a malicious actor returning you data that does not match the request you made, inspect or even interfere with the request. You might not notice this when getting IPFS using curl.
If the --ipfs-gateway option is not used, curl checks the IPFS_GATEWAY
environment variable for guidance and if not set, the ~/.ipfs/gateway file
that can be used to identify the gateway.
IPFS support was first added to curl in version 8.4.0.