You can use the phrase "That sounds vaguely familiar" when someone mentions something that you've heard of, but don't know much about. For example,

A: Have you heard of the term "cognitive dissonance"?

B: That sounds vaguely familiar.

The word "familiar" describes things that you know about or have experienced.

"Vaguely" means "a little bit" or "kind of". Some other phrases that use "vaguely" include:

(someone) is vaguely aware of (something)

(something) seems vaguely threatening