You have seen a dragon what does it mean?

That depends. Most likely, it means that you just need to wake up and get out of bed.

However, someone who is superstitious or believes in signs given in the form of visions could claim that it is a sign from a higher dimension.
Dragons are often known for being powerful, fearsome and undefeated by many. They are also known for being the evil being hated by many. They can be viewed as greedy, since dragons in literature often hoarded a treasure. Thus, if you believe that it could be a sign, you could take it to mean that you should avoid being greedy, or that you should beware of others who are greedy. It could mean that you should beware of an enemy (the knight in shining armor.) Or it could mean that you are being watched over, like a dragon watches and guards his treasure.

There are many "meanings" that could be read into this, but you should not be too hasty to believe all of it, as some people would read something totally ridiculous and impossible into it.