Hello Vi Vi, it's just another way of saying, "You are right." So, it's something people say when they want to express agreement. Hope that help!

It also means that they understand what you are going to do and, as Olivia says, are in agreement. "I'm going shopping!" "Right you are!" The person answering has acknowledged that they understand the first person is going shopping, they also have a chance to interact with whatever the first person is doing. They may even add something. "In that case can you take my car and fill it with petrol (gas)"

I think this expression is used more in British English than in American English. Personally, I've never used this phrase in my life.