The baseline audience is what your audience is being compared to. Your baseline audience by default is the Online Adult Population.
You can change your baseline audience to any previously saved audience you like. For example, a Female Apparel company may prefer a female-only audience as the baseline rather than the entire Online Adult Population, which of course includes men. Once they've changed the baseline to be females only, they'll be viewing insights under the lens of what all women think, rather than compared to what everyone thinks.
Read more about use cases on how best to change your baseline.