you’re talking, you know얘기하고 있다 means “to be talking,” while 잖아 adds emphasis or reminds the listener of something assumed to be known. Depending on context, it can also mean “I’m talking, you know” or “I told you, we’re talking.”
KO: 나 지금 중요한 얘기하고 있잖아.
EN: I’m talking about something important, you know.