HTML (HyperText Markup Language) is the fundamental language of the web. It's a declarative language rather than a programming language, and also the web's most important markup language—basically, a text file with "markup" baked into it to explain the specific nature of various bits of text.
CSS (Cascading StyleSheets) is another declarative language: it's a flexible, powerful, repeatable way to style—control the appearance of—HTML markup.
PHP (PHP: Hypertext Preprocessor) is the most important language to truly understand WordPress. PHP is a programming language that runs on a web server, the machine that hosts a website. PHP can perform all sorts of dynamic operations, and outputs HTML to send to the client's browser at the end. PHP is the core language of WordPress: WordPress core itself, and nearly all WordPress themes and plugins, are primarily written in PHP.