Apache Tomcat is an open-source server for Java servlets. If you need to serve some java apps you should take a look to the newest Tomcat 9. It’s easy to use and set up.
What is TLS?
In short it’s a protocol that provides a secure connection between host and client. You can read more about that in Wiki.
In what cases it may be useful?
You can meet it in case of work with secure connection. The most primitive example is reading content into a string from website which works with TLS. It may be required when you are working with API.