As far into HTML as you now are...I'm sure you have heard the term "JavaScript" in your HTML travels.
JavaScripts are written using the Java programming language. They are like little pieces of java that you can add to your pages to make them more interactive...fancier.
You do not have to know java to have javascripts on your page! You don't have to know JavaScript either. You just have to know how to copy and paste.
You can find many good, useful scripts online...for free...that can be copied and pasted into your HTML documents that will work just fine...you just have to know where to look.
Most of the places where you can get JavaScripts will give you instructions as to exactly how to run the scripts on your page...just read the directions.
JavaScripts have many uses in a web page. They can be used to create calendars, scrolling text, fancy text, disappearing text. They can be used to password protect some or all of your pages, gather information about your visitors, and much, much more.
You can learn more about writing your own JavaScripts from a simple web search on Google