An HTML encoder is a tool that helps to convert plain text into HTML code. HTML encoding is the process of replacing characters with their equivalent HTML entities, which are special codes used to represent characters that may have special meaning in HTML.

HTML encoding is used to ensure that special characters such as "<", ">", and "&" are interpreted correctly by web browsers and other software that processes HTML code. For example, if you want to include the symbol "&" in your HTML code, you would use the HTML entity "&" instead.

An HTML encoder tool typically provides a simple interface where you can input your text and get the encoded HTML output. Some encoder tools also allow you to choose which encoding standard to use, such as HTML4, HTML5, or XML.

Overall, HTML encoder tools are useful for web developers and content creators who need to ensure that their HTML code is valid and interpretable by web browsers and other software.