HTML encoding is the process of converting special characters and symbols into their corresponding HTML entities. This is necessary because HTML has a set of reserved characters and symbols that have special meanings, such as < and > for tags, and & for entity references. If you want to include these characters in your HTML document without them being interpreted as HTML code, you need to encode them.
For example, the less than symbol < should be replaced with the entity reference <, and the greater than symbol > should be replaced with >. The ampersand symbol & should be replaced with &. Other common entities include for a non-breaking space, © for the copyright symbol, and ® for the registered trademark symbol.
HTML encoding can be done manually by replacing each special character with its corresponding entity reference, or automatically using an HTML encoding tool or library. It is important to always use encoding when working with user-generated content or data from external sources to prevent security vulnerabilities such as cross-site scripting (XSS) attacks.