Could you explain the difference between "I'm for real" and "I'm serious" in this situation?

Rebecca

The difference between "I'm for real" and "I'm serious" is that "I'm for real" is a slang, informal term, and "I'm serious" is a proper term. "I'm for real" is used here because the speaker is imitating a dialect in the United States that uses this term. I recommend using "I'm serious".