Secure Elasticsearch installation retrospectively
The elastic bootstrap password is used to init the internal/reserved users used by the components or features of the elastic stack (kibana, logstash, beats, monitoring, ...).
If you want to secure the API, you need to create users/roles for your scenario on top.
Please use TLS in your cluster when handling with passwords and don't expose the cluster directly for security reasons.
Here are all informations regarding a secure cluster including some tutorials: https://www.elastic.co/guide/en/elasticsearch/reference/7.3/secure-cluster.html
EDIT: Added links as requested. Feel free to raise a new question here at SO if you're facing serious problems!
Here you can find a complete guide to install and secure ElasticSearch.
Basically the bootstrap password is used initially to setup the built-in ElasticSearch users (like "elastic", "kibana"). Once this is done, you won't be able access ElasticSearch anonymously but only with one of the built in users, e.g. "elastic".
Then you can use "elastic" user to create additional users (with their own password) and roles (e.g. to asses specific indexes only in read-only mode).
As @ibexit wrote it's highly recommended to secure your cluster and don't expose it directly (use a proxy server, secured with SSL).