|The Simple English Wiktionary has a definition for: undercover.|
Being undercover means that a person is wearing normal clothing and is hiding their identity from people. It is often used by police officers when they are investigating crimes or criminals. It is done to gain trust and also possibly to gather evidence. It is not the same as a police officer who is wearing normal clothing (a plain-clothes officer) but who is not hiding their real name and that they are a police officer.