Sub-rights are a great way to bring in more money and get more exposure for your book. If you are self-publishing then you have the responsibility to see if these rights can be sold. If you land an agent or a publisher, they should be taking care of these rights for you. An agent will be shopping these with your interests in mind, the publishers in their own interest. There are a bunch of subsidiary rights that you may not know or care to know about, even after you've published, but there are 3 major ones that you should be asking about before you sign anything.
This is the obvious one. Most writers have already dreamed of that actor that will play their characters in a film.
Audio sales have seen a steady increase over the years, so much so that publishers are more often trying to retain these. With the advent of digital content streaming, companies such as Audible are carving out a place in the market.
Where in the world and in what language your book is being published is determined by which rights you have signed away. Did you give the publisher only North American English or did they retain World English? What about the right to translate and sell it in different countries?
If an agent has offered you representation, ask what their process is for retaining and selling these important rights. How does the commission percentage break down? Do they have a sub-agent or do they partner with an agency that specializes? If the publisher insists on keeping one or all of these rights, what sort of parameters does the agent set in order to make the deal worth it? Most agents should be able to answer these questions easily and happily.
If you went straight to a publisher, make sure you understand what is happening with these rights. If said publisher is keeping all these rights, there should be a time limit so if the publisher does not doing anything with these rights they revert back to you.