Resolution and what it means to derive the empty set
To complete the Kyle's answer:

A CNF (in Conjunctive normal form) formula, $F$, can be considered as a set of clauses, eg : $F=\{C_1,C_2 \ldots C_m\}$ and each clause can be regarded as a set of literals, eg : $C_1=\{x,y\}, C_2=\{\lnot x, \lnot y\}$.

If $F=\varnothing$ i.e. $F$ is the empty set and does not contain any clause then $F$ is satisfiable (=tautology).

If $F=\{\varnothing, C_1 \ldots\}$ i.e. $F$ contains the empty clause then $F$ is not satisfiable. The empty clause can be obtained by the resolution rule eg : the resolvent of $C_3=\{x\}$ and $C_4=\{\lnot x\}$ is the empty clause $\varnothing=\{\}$.