A ringer can mean a few different things:

1. Someone who pretends to be less skilled than they really are so they can compete unfairly and win easily. If your friend's “cousin who's never played before” shows up to your basketball game and starts making perfect three-pointers, you might suspect they're a ringer. Today, you might encounter ringers in amateur sports leagues, video game tournaments, or talent competitions. The key is deception: a ringer hides their true ability to gain an unfair advantage.
2. Someone or something that looks almost exactly like another person or thing. If you see someone who could be your teacher's twin, you might say they're a “dead ringer” for your teacher. When people are dead ringers for each other, they're so similar that you might mistake one for the other.

Both meanings share the idea of substitution or similarity: someone or something that isn't quite what it first appears to be.