An encoding defines a mapping between bytes and text. A sequence of bytes allows for different textual interpretations. By specifying a particular encoding (such as UTF-8), we specify how the sequence of bytes is to be interpreted.
For example, in HTML we normally declare a character encoding of UTF-8, using the following line:
<meta charset="utf-8" />
This ensures that you can use characters from just about any human language in your HTML document, and they will display reliably.