Have you heard this phrase as a response to a question like "What do you think?" or "Is that okay with you?"

"I'm down" is a casual way to say "That sounds good," "It's cool with me," or "Let's do it."

A student asked me about this phrase a few days ago because she thought it meant "I'm sad," or "I'm feeling down."

As with many English expressions, context is everything! "I'm down" is a positive, energetic phrase. You might also hear "I'm down for that," or even "I'm up for that."