A variation of the transport layer security (TLS), there is mutual TLS (mTLS). TLS is the successor to secure sockets layer (SSL) and is the most widely deployed standard for secure communications, most notably for HTTPS.
TLS establishes secure communications that are both confidential and resistant to eaves dropping as well as authentic and resistant to tampering between a server which needs to prove its identity to its clients.
When there is a situation where both server and client need to prove their identity to each other, such as a Kubernetes application setup as microservices, TLS is not enough.
mTLS must be used in such use cases where both server and client need to prove their identity. It is an extension of TLS as it adds mutual authentication between the client and the server.