It means: "What are you looking at". If you insert the phrase "the hell", it just makes the question stronger and more demanding. It is also offensive. It indicates that you are angry. You can use it in many sentences, and it goes before the verb, for example: "Where are my shoes?" ----> "Where the hell are my shoes?" "What do you want? ----> "What the hell do you want?" "How did you find me?" ----> "How the hell did you find me?"

"What the hell are you looking at?"= an angry question to someone who is staring at you rudely. Example: A man kept staring at the young woman and her friends in the nightclub. She said, "What the hell are you looking at?" The man looked away and did not stare at them again.

It's a rude way of confronting someone that is looking at you. For example, if you are on the train in a really bad mood and someone across from you keeps staring at you, you might look him in the eye and say "what the hell are you looking at?". Keep in mind this is quite rude and you are probably better off not using it. Of course, literally it just means "what are you looking at", but with extra emphasis. Technically the other person doesn't have to be looking at you to use it, but as a phrase that's usually what it means.

Only say it when someone is looking at you or something you own in a very rude way.

Many English speaker don't use 'hell'. I forgot the word the most often use... I think it was something similar to 'duck' or 'luck'... but I think it started with 'f'. Either way, don't use it. It's offensive.