An idol can be a religious image or a person who people admire and maybe even seem to worship. A statue of a Hindu god is a religious idol, but Madame Curie is an idol to aspiring scientists.
The word idol comes from the Old French idole for "pagan god," through the Greek eidolon for “reflection in water or a mirror.” In religion, an idol isn’t the real deity but a representation of it. These days, rock stars and celebrities are idols, but so are scientists and writers. If you say Flannery O’ Connor is your idol, you don’t mean you bow down before a statue of the Southern writer, it just means you admire her big time.