"I'm down" could mean: I'm depressed, I'm feeling blue.
"I'm down with it/that," could mean "I agree," "I understand what you said."
"I'm down here" could be something you might yell in response to someone who is calling your name, assuming you are in a basement or other location where you are "lower" than the person calling you.
As you can see, there are several possibilities. Can you tell us which is closest, and we can provide further explanation?
P.S. Please see our Rule #22:
Members must do their best to write using standard language forms.
We might note that this phrase may be "used all the time" among people of a certain generation. It seems to me that its use in this context is more popular among people in their late teens/early twenties.
I would agree with GenJen about the liikely demographic use of "I'm down."
Without any context or knowing anything about the person using the phrase I would assume it was meant as 'I am depressed or sad.'
There are a few other meanings -
A gambler might say "I'm down 100" - I have lost $100.
A stock market trader might say "I'm down 100" - my stock has lost $100 in value or 100 points on the Stock Exchange. Another trader might ask - 'How are you doing?' You answer "I'm down" meaning your stocks have lost (an unspecified) value.
In AAVE "I'm down" or "I'm down with that" can mean "I am in", I will commit to that action or position, or I am on your side.