The \- command tells LaTeX that it may hyphenate the word at that
point. LaTeX is very good at hyphenating, and it will usually find all
correct hyphenation points. The \- command is used for the
exceptional cases.
Note that when you insert \- commands in a word, the word will
only be hyphenated at those points and not at any of the hyphenation
points that LaTeX might otherwise have chosen.