Use this casual expression when you're at the end of a conversation and you don't have anything else to say. For example, if you're introducing yourself to a group of people in a meeting at a conference, you might end your introduction this way:
So, yeah. That's about it.
It means that you don't have anything else to say about yourself.
"So, yeah." makes it sound like you've tried to think of other things to say about a topic, but you can't. So when you're trying to sound confident and intelligent, it's probably better not to use "So, yeah."