How to get cacert for enablingssl_certificate_verification?

ACTUAL TITLE OF THIS QUESTION: How to get cacert for enablingssl_certificate_verification?
The validation says: Title seems unclear, is it a complete sentence?

It may be a very silly question, but I couldn't resolve it spending many hours.
Wanted to set output.elasticsearch.enablingssl_certificate_verification.
"If ssl_certificate_verification is true, uncomment cacert and set the path to the certificate.".
I have no idea from where I will get certificate for that. I don't understand exactly why a client need certificate.

The comment "If ssl_certificate_verification is true, uncomment cacert and set the path to the certificate." was in
elastiflow.

Raised issue there: