TLS client certificates are a way for clients to cryptographically prove to servers that they are truly the right peer (also sometimes known as Mutual TLS or mTLS). A command line that uses a client certificate specifies the certificate and the corresponding key, and they are then passed on the TLS handshake with the server.
You need to have your client certificate already stored in a file when doing this and you should supposedly have gotten it from the right instance via a different channel previously.
The key is typically protected by a password that you need to provide or get prompted for interactively.
curl offers options to let you specify a single file that is both the client certificate and the private key concatenated using
--cert, or you can specify the key file independently with
curl --cert mycert:mypassword https://example.com
curl --cert mycert:mypassword --key mykey https://example.com
For some TLS backends you can also pass in the key and certificate using different types:
curl --cert mycert:mypassword --cert-type PEM \
--key mykey --key-type PEM https://example.com