The Minimum word frequency is normally used to determine which words are in the Word Section of the glossary. It also influences the selection of which phrases appear in the Phrase Section.
In some cases, you may want to include all words of a document in the Word Section of the glossary, even words that appear only once. For example, you may be compiling a glossary from a document that defines a list of long technical words, and you may want all of them to be collected. In such cases, the Minimum word frequency is only used to influence phrase selection.
See the Rules for Phrase Selection for a discussion of this subject.