Learn Danish: Du har ret. - You are right.

In Danish, you don't 'be' right, you 'have' right. It is a set phrase: at have ret literally translates to 'to have rightness.' If you try to translate it directly and say Du er ret, it sounds like you are saying 'You are straight' or 'You are quite...' (leaving the thought unfinished).

Is ret an adjective here?

No, in this phrase ret acts as a noun. That is exactly why it pairs with the verb har (have). You are literally saying that you possess correctness.

Since Du har ret means 'You are right', can I say Du har fejl for 'You are wrong'?

No, Danish uses a different set phrase for being wrong. To say 'You are wrong,' you say Du tager fejl, which literally means 'You take mistake.'

I have seen the word ret used in other ways. What else does it mean?

ret is a very common word with multiple meanings depending on the context. Besides 'right' (correctness), it can mean a 'dish' (as in food), 'straight' (as in a line), or 'quite/rather' (like saying ret stor for 'quite big').