What is an Ex Libris? In short: "Ex Libris" means "from the Library of". It is a designed bookplate that bears the name of a book's owner.
In long: This book plate makes it very clear that a book is part of YOUR library (avoiding confusion or possibly theft, I would imagine) It is usually found in the front inner cover or first blank page-- placed there either with adhesive or stamped in. Nowadays, we more often see these types of bookplates and simpler identifiers in library books and books that are part of large official collections. But, it was at one point not so uncommon for individual readers to have them, too! There's a rich history of really beautiful and creative ex libris designs commissioned by individuals and book collectors. This is a great way to show your personality and interests as a reader! They can be as elaborate or as simple as you like. (Just saying, it also makes a thoughtful gift to anyone who reads a lot!)