Yup.

Hello, It is equal to "i am serious". It refers to something that was already started, like a command, and you are now putting the an exclamation or emphasis on your statement.

I mean it = I'm serious. 'Really' can also be used, but 'I mean it' carries more weight. If you want to make it clear that there are serious consequences to what you are saying, you might also say: "I'm dead serious" You might think that "I'm deadly serious" would be used, but unless someone's life is in danger, it would be much too overly dramatic.