When something is labeled, it has a marker that clearly identifies whose it is or what it contains. If your labeled suitcase gets misplaced, it has a better chance of being returned than a bag without a tag.
Vintage clothing that's labeled, especially if it's made by a well-known designer, will often cost more than sweaters and dresses with no tag or label. To be labeled in this way is to be clearly marked with a brand (and often a size, fabric information, and laundering instructions). The opposite of labeled is unlabeled.