The resources I've come across mention that
When the two clauses contain more than one pair of complementary literals, the resolution rule can be applied (independently) for each such pair; however, the result is always a tautology, and thus discarded.
As I understand it, this statement means
$A \vee B, \quad A \vee \neg B \Rightarrow A$ is the right resolution
$A \vee B \vee \neg C, \quad A \vee \neg B \vee C \Rightarrow A$ is not
However I don't really get what it means. I think I understand what is a tautology, however how does the result of such a resolution step become a tautology, and why do we have to discard it instead of adding it to the set of clauses? More detailed explanation isn't provided in the resources I've found.