The difference between these two theories has to do with whether a society will come to be homogeneous or differentiated as it absorbs people from different cultures.
The meling pot model assumes a great degree of homogenization. According to this theory, the various cultures interact with one another and are all changed. They become one, uniform, culture which has aspects of the various cultures from which it was made.
The cultural pluralism model assumes a much lesser degree of homogenization. According to this theory, the various cultures will not melt into one another but will, instead, remain fairly separate. The society will end up, for example, with many different languages, religions, and cultures. It will be a society that is diversified rather than one that is homogeneous.