Web Development is the art of writing code that web browsers can understand. The core technologies of web development are:
HTML - HyperText Markup Language - is the language used to describe the content of a website. All of the headings, text, images and links you see in many web pages are written in HTML. HTML was originally developed by Tim Berners-Less at CERN, the European Laboratory for Particle Physics in Geneva, Switzerland. It was originally meant for use in scientific research documents.
CSS - Cascading StyleSheets - is the language used to style content. For the most part, HTML should be used to add new content to a site, and CSS should control the presentation of that content. In the not-so-good-old-days of the 1990s, web pages were often laid out in tables, which is a type of HTML content. This practice has gone the way of the dinosaur, and it is universally recognized that page layout should be controlled with CSS. Dividing content and styling like this is called a separation of concerns.
CSS was created by the W3C - the World Wide Web Consortium in 1996. It is a technology that is simple to learn but difficult to master. If you find the process frustrating just know you are not alone!
There are many other programming languages and technologies that are important in web development, but none so much as the core languages described above. New developers should focus on mastering those three before moving on to others.
There is literally no skill better supported on the Internet than learning web development. Check out this great list of resources to get started!