Adapted from the really fantastic Team Treehouse blog A text editor is a computer program that allows you to create and edit files that contain plain text. This is very different than a robust word processing program like Microsoft Word, Apple Pages, or Google Docs, because those programs add special formatting and information to the file that's hidden to the user (think things like fonts, formatting, spacing, etc). When you're writing computer code, you don't need or want any special formatting to be part of the file. In fact, your code probably won't work at all unless it was written and saved in a plain text environment.
Text editors are commonly used by web designers and developers because it provides maximum flexibility to create just about anything. This is where professionals "hand code" HTML, CSS, JavaScript, Ruby, PHP, and many other languages.
For completeness, I should also point out that a text editor is not the same thing as an IDE (integrated development environment). While almost every IDE has the capability of creating and editing plain text documents, they also include a wealth of other features and powerful capabilities. Some people prefer _IDE_s, but they can sometimes come at the expense of other factors like flexibility, ease of use, or code that is both lean and performant. A more everyday text editor lacks these capabilities in exchange for maximum flexibility and ease of use.