In most cases people does not really want to use SSL directly in Reposilite, because they serve it behind some proxy services like Nginx and usually that's where they should set it up.
If you're not sure how to bootstrap your infrastructure and SSL, take a look on the official Javalin SSL guide:
JKS stands for Java KeyStore format.
You may find various resources related to this topic.
Take a look at e.g. on Jenkov's blog or directly in Javalin's/Jetty examples: