Very good question! I expressed this thought to some people a week ago. People nowadays say “this is a right” and “that is a right” but when you ask them “What is a right?”, I’m guessing they wouldn’t have a good answer!
I’m interested to know what others have to say.
This article might be helpful which might give you ideas as to what to search for next, or possibly, other sources you might want to consider looking at:
A good start might be studying Aquinas. He’ll discuss the Eternal Law, Natural Law, and Human Law. Although studying the Summa and his works may be intense. So a summary of Aquinas may be helpful. There are books that go over each part or certain parts of his works, if I am not mistaken.
To understand what a right is, you also have to understand what is right and what is wrong. There are many factors that justify what is a right and what is not (such as theology, the Church’s teachings, etc.), but I’m interested in how your studying this will go for you! I think that even reading Church documents such as Dignitas Personae or Humanae Vitae will give you a feel for how it is they come to what is a right and what is not. If you can get the feel for it, you’re a step farther!