HTML, or HyperText Markup Language, is the basic language of the internet. HTML is essentially a text file explaining the unique nature of certain sections of text.
CSS, which stands for Cascading StyleSheets, is another declarative language. It's a versatile and powerful method for styling the look of an HTML file.
PHP, or Hypertext Preprocessor, is the most necessary language to really grasp WordPress. PHP can conduct a wide range of dynamic operations and generate HTML to deliver to the client's browser.