According to traditional grammar rules, them is actually an object pronoun, not a preposition. It's used as a direct or indirect object in a sentence, like "I gave them a gift." But, some linguists argue that them can be used as a preposition in certain contexts, like "I'm going with them to the movies."
This is where things get interesting! Some dialects of English, like African American Vernacular English, use them as a preposition more frequently than others. It's a great example of how language is constantly evolving and adapting to different cultures and communities. Language is messy, and that's what makes it so fascinating!