What does HTTP 422 'Unprocessable Entity' signify in an API context?

HTTP 422 'Unprocessable Entity' indicates that the server understands the request format, but was unable to process the instructions contained within due to semantic errors. This often means the data provided doesn't conform to the required validation rules, such as missing required fields, incorrect data types, or values outside acceptable ranges. It's distinct from 400 Bad Request, which generally indicates a problem with the request format itself.