An HTML Decoder tool is a program or website that can decode HTML entities, which are special characters used in HTML code to display symbols or characters that are not typically available on a keyboard.

HTML entities are used to represent characters such as <, >, &, ", ', and other symbols, as well as non-English characters and mathematical symbols. When writing HTML code, these characters must be encoded using their corresponding HTML entities, or the code will not be interpreted correctly by web browsers.

An HTML decoder tool can take a string of text that contains HTML-encoded entities and convert them back into their corresponding characters. This is useful when working with HTML code and when copying text from web pages that may contain encoded characters.

Some HTML decoder tools are available as standalone programs, while others are web-based tools that can be accessed through a web browser. These tools typically provide a simple interface for entering the encoded text and viewing the decoded output.