This module allows you to associate multiple vocabularies to a term-reference field. Standard drupal term reference fields allow you to reference terms in a single vocabulary only.
Use case
Let's say you have a free tagging field on a node which your editors would like to use to tag content with. They would like to tag content with terms from different vocabularies. For e.g. content could be an article of type Insight, and be also tagged with category philosophy. It could further be tagged with a personality Freud. In this case, these three are terms from different vocabularies.
Insight - is an Article type term.
Philosophy - is a Category term.
Freud - is an unassociated random Tag.
Article type, Category and Tag are vocabularies.
It makes sense to have a term reference field referencing these vocabularies instead of just one to avoid creation of duplicate terms. In the present case (without multiple vocab term reference), all terms would get stored into the Tag vocabulary. There would also be duplicate terms in the Article type & Category vocabularies. This would also lead to term URLs like /insight & /insight-0 if you are using pathauto to alias your term pages with the term name.