spread is a pair of pages in a book or magazine, the two you see when you've opened the book to read it. The term is used to describe both the left-hand and right-hand pages as a unit, or one thing.
If you ever paint illustrations for a children's book, you'll hear the publisher and book designer talking about spreads. Books are planned as a series of spreads (rather than individual pages) because it's important to visualize how the pages that will be open together will work against one another.
The term 'spread' is also used to describe the very first and very last page in the book, where as an illustrator you need to consider what appears on the inside of the cover (or end papers, if they're being used.)
