A remainder mark shows that a book was sold off cheaply by the publisher as surplus stock, known as a remainder. To stop remaindered copies being returned to shops for a full refund, the publisher or wholesaler marked them, usually on the bottom edge of the pages.
How to spot it
Turn the book over and look at the bottom edge of the text block, the edge opposite the top of the page. A remainder mark is usually a short line or dot of marker pen, a small spray of ink, or a rubber stamp such as a star or the publisher's initial. Sometimes it is on the top edge instead. Some are faint, so tilt the book under a light.
What it does to value
A remainder mark is a minor fault, but collectors note it, and dealers always mention it. On a modern first edition in fine condition it can lower the price a little, because a clean, unmarked copy is easy to find for most titles. It matters more on books that sold poorly at first and later became sought after, because many surviving firsts of those titles are remainders.
A remainder mark does not mean the book is a later printing: remaindered copies are often first printings that simply did not sell. See book condition grading for how marks are listed.