If you want to sell books to a royalty-paying publisher, you probably need a literary agent. But first a few words about royalties.
A publisher contracts with you for your book, edits it, produces it, and distributes it and usually pays you a small percentage based on sales. That part is easy to grasp.
What isn't simple is the way publishers figure royalties. Some base the royalty on the suggested retail price, others on the wholesale price. Publishers grant large discounts to Walmart, Costco, and other nonbookstore outlets. It's not uncommon for them to offer those outlets a 75 percent discount on the retail price. When that happens, the authors' royalty rates are lower. (And it's all in the contract.) Regardless, they pay royalties based on the net sales. That is, the publisher expects returned books, so it's not paid only on books sent to outlets.
Another factor is that sometimes publishers grant escalator clauses. That is, once a book has sold a certain number of copies, such as 50,000 or 100,000 they offer a bonus. (When my agent negotiated the contract for 90 Minutes in Heaven, she, Don Piper, and I believed the book could become a big seller, but the publisher didn't. So with my agent's help we received escalator clauses when the book reached certain sales figures.)
The royalty rates vary on hardcover, soft cover/trade, and mass paper. Some publishers, especially the large ones, pay royalties each quarter, some twice a year, and most of the smaller houses send out annual checks. They also send statements of sales—that's not always easy to follow and agents can decipher and demand clarification. That's another good reason to have an agent represent you.
Serious writers who sell books need literary agents to negotiate royalties.