Semantics can be defined both as the study of the meaning of language or as the meanings and interpretations of individual words.
As it applies to argumentation, semantics should be considered in two ways.
First, one should avoid making purely semantic attacks. If a person's word choice does not entirely match their intention, but the argument they make is clear, then semantics should be overlooked. Nit-picking in such a way is counterproductive. It creates the impression that one is unable to validly counter the argument.
Second, one should try to consider semantics regarding word choice. By using the clearest words possible, confusion can be avoided. When semantics are a priority in the preparation of an argument, that argument becomes much more effective through its clarity. Along these lines, if a presendted argument is ambiguous or clearly contradicts logic in an unusual way, one should question semantics. This differs from making a semantic attack.
An example would be the use of the word "moral." In one view, morality is inherently religious, defined by the decree of a god. One can also use the term in a secular fashion to describe a proper method of behavior independent of gods. The difference is very important when arguing the morality of a god or of said god's followers. Before entering into a debate over a god's morality, it is a good idea to make it clear to the other party exactly what is to be meant by the word "moral" to assure clarity and to avoid opening the argument to semantic cop-outs ("killing adulterers is moral because God commands it").