Throwaway lines are often one-liners, or in-jokes, and often delivered in a deadpan manner.
Similarly, in theater, a throwaway line is one uttered by a character where the only intended reaction is that of from the audience.
Oftentimes, these lines may be references to other shows or media that only the audience are aware of.
This is different from breaking the fourth wall because the line is not directed at the audience; however the other characters onstage will generally proceed as if nothing has been said, or that nothing of value was said.
[1] As an example, in the second season of the American television series Breaking Bad, the character Saul Goodman, after being kidnapped and threatened at gunpoint by a masked Walter White and Jesse Pinkman, tries to defuse the situation by blaming an unseen person named "Ignacio" and referencing someone named "Lalo".