Ask any Supernatural fan what the shows about, They will all say similar things. They will say it's a toon about two brothers that hunt demons, ghosts and your worst nightmares. They may include some details about castiel of the metallicar. But there still very alike.

But if u look deeper u will see that past all the heaven, hell, angels and deamons. That it's about to brothers. SAM AND DEAN. To brothers that will do anything for each other. From the first to the last episode in every series so far. Dean has been protecting Sam.  And Sam has been looking out for dean. 

Now in most episodes u just see them fight a few monsters, save a few people, and make some friends. They drive away and never look back. But in a few episodes, if your lucky,  you get to see behind the masks of Sam and dean, and see how they really feel. u see how deans always felt he had to protect Sam from everything, and let him have a normal life. And how Sam has always looked up to dean. 

Ask Supernatural fans what the shows about, they will all say similar things. And they're right. The first thing they say is: "it's about two brothers." Two brothers that will fight through heaven an hell for each other. 
And that's all it's really about