A Data-Only Blueprint is a Blueprint Class that contains only the code (in the form of node graphs), variables, and components inherited from its parent. These allow those inherited properties to be tweaked and modified, but no new elements can be added. These are essentially a replacement for archetypes and can be used to allow designers to tweak properties or set items with variations.
Data-Only Blueprint are edited in a compact property editor, but can also be "converted" to full Blueprints by simply adding code, variables, or components using the full Blueprint Editor.