Resolution
Resolution is a way to make a new clause from two existing clauses. If the two original clauses were true, then the new one is, too.
The two existing clauses must be compatible, in order to use resolution. There must exist some literal in one clause that occurs negated in the other clause.
Example: Clause 1: A V ~ B V C
These are compatible, because the literal ~B in Clause 1 occurs negated (as B) in Clause 2.
These two clauses have a unique resolvent, since there is only one complementary literal, B.
The resolvent is formed by disjoining (i.e., combining with "V" all the literals of clause 1 and clause 2 except the ones involving B.
This results in A V C V D.